Tuning Weber IDA Carburetors

This section is currently under construction. Please bear with me.


This section is dedicated to the tuning of the venerable Weber IDA downdraft carburetor. For those of you who may not be familiar with this carb, it is a carburetor that was designed primarily for racing and it has a rich racing heritage. The IDA may be best known for its use in the Ford/Shelby race cars of the 1960s. They were often used for the 289 FIA Shelby Cobra, the Shelby Cobra Daytona Coupe and the Ford GT40. They were also common in many formula cars. Indeed, it is somewhat iconic of 1960’s open-wheel cars to have an American V-8 behind the driver with 8 stacks poking up out of the engine. Those stacks were invariably Weber IDA carbs. They are also often used as a performance upgrade for air-cooled VWs and a three-barrel variant designated the IDA3C is often used as an aftermarket go-fast goodie for Porsches. The IDA3C variant was also often used on Lamborghini and Ferrari 12-cylinder engines. I was exposed to the IDA by way of the replica of a 1965 Shelby Cobra that I built. In the interest of authenticity, performance and the “wow” factor, I decided to equip the car with Weber IDAs.

The Weber IDA is a remarkable carburetor. It is a two-barrel progressive carb that is almost infinitely adjustable. Depending on how you do the counting, there are 10 or so individual parts that are available in different sizes for tuning. As we will see later, this is both a blessing and a curse. All of these adjustment parameters mean that there is almost certainly the perfect combination for every motor. These carburetors are just as at home servicing a 1500cc VW motor as they are atop a 460 cubic inch Ford FE. Of course, the complexity means that finding that perfect combination can be difficult. A well-tuned motor with IDAs will have exceedingly crisp throttle response and mountains of torque that arrives early in the rev range. A motor that is not properly tuned can be an intractable monster that spits and coughs, breathes fire and consumes huge quantities of fuel.

So, what makes Weber IDAs so great? Well, as already mentioned, they are almost infinitely adjustable. But that is only part of the story. They are also manufactured to the highest quality standards. The throttle shafts roll on ball-bearings. The accelerator pump uses a precision brass piston riding in an equally precise bore rather than a rubber diaphragm (except for the IDA3C which uses a diaphragm pump). By all accounts, they are exquisite pieces of engineering art. But even that isn’t the full story. The most common application of the Weber IDA is in a V-8 motor using an independent runner manifold. What this means is that each cylinder essentially gets its own personal carburetor, and that carb is just a couple of inches away from the valves, connected by its own manifold runner. When the intake valve opens, there is only a small column of air that has to move to deliver fuel and air to the cylinder. Compare this to a conventional 4-barrel arrangement. Each cylinder is connected to a common plenum, and the carb can be as much as a foot away from each cylinder. When a cylinder valve opens, the entire column of air reaching back through the manifold back to the carb must begin to move, and that takes time, which affects throttle response. Furthermore, that cylinder must pull air and fuel from a carb that is designed to deliver air and fuel to 8 cylinders, not just one. Contrast that to IDA, which provides a dedicated barrel to each cylinder and can therefore be optimized for that cylinder rather than compromising in order to be able to serve 8 cylinders.

There are some common misconceptions about Weber IDAs. I recall when I was designing the motor for my Cobra replica I was talking to a gentleman who is very highly regarded in the industry about my carburetion options. I told him that I was considering Weber IDAs. He was very adamant that the IDAs were a poor choice and went on to explain why. For my application (a 331 stroker small block Ford) the “traditional” carburetor might be a Holley 650 CFM four barrel. This carb has 4 bores that are approximately 43mm in diameter. A Weber IDA system for the same motor might use 8 bores that are maybe 40mm in diameter. He explained that the Webers would fall flat on their faces when nailing the throttle because they would suddenly be opening up nearly twice the throttle area of the Holley 4-barrel. This, he reasoned, would kill air velocity and therefore performance.

At first glance, his reasoning seems sound. But it’s actually not, and the reasons become clear if you think about it a little bit more. A motor demands air only one cylinder at a time. In a traditional 4-barrel setup, the carb is essentially only serving one cylinder at a time, and each cylinder “sees” all 4 barrels plus the entire manifold at WOT. In essence, this is just the opposite of what the expert explained to me. An engine with conventional carb sees far more area when the throttle is nailed than an engine with Webers. Consider an IDA on an individual runner. Each cylinder only “sees” its dedicated barrel and a very short manifold runner. It certainly doesn’t “see” any of the other independent barrels. This results in a very fast-moving air charge. Torque is a highly dependent on air velocity and high velocity increases torque. Since the runner is so short, the air can get moving very quickly, increasing throttle response. There is a reason that the Weber IDA on an independent runner manifold was the carburetor of choice for race cars prior to the advent of fuel injection. It provided the best torque, and throttle response – essential for a race car.

First Things First:

Before we begin the tuning process, there are some things that must be considered and some preparatory tasks to perform. In this section, we’ll talk about camshaft selection, timing, fuel pressure and supply, setting the float level, synchronization, and linkage.

Camshaft Selection:

Volumes have been written on camshaft theory and I won’t write a complete treatise here on the subject. However, there are certain considerations that are important regarding Webers, and I will touch on those in this section. If you are building an engine to work with Weber IDA carbs, camshaft selection is very important, and you must approach the selection of a camshaft differently than you would with a shared-plenum carb setup. In general, a camshaft is designed to increase volumetric efficiency (VE) for a given performance range. This is done using two very basic premises – increase the amount of intake charge that enters the cylinder, and increase the amount of spent exhaust gases leaving the cylinder. An engine with a mythical 100% VE will completely fill the combustion chamber with the pure intake charge on the intake stroke, completely burn that intake charge on the compression and combustion strokes, and then completely void the exhaust on the exhaust stroke.

However, this never works in practice. During the intake stroke, when the piston is traveling down in the cylinder and the intake valve is open, the vacuum created draws fuel and air into the combustion chamber via the carburetor. In a perfect world, there are no restrictions to the flow of the fuel-air mixture, the gas mixture has no inertia that inhibits it’s ability to get moving and the chamber is able to fully fill. In practice, that is not the case. Even under the most ideal conditions, there is a very limited amount of time to get the chamber filled, and any obstructions in the path between the carburetor and the combustion chamber (like intake valves, valve openings, throttle plates, etc.) will impede the flow of the mixture. Additionally, gas has inertia, and it takes time and energy to get it moving. This causes the combustion chamber to fill incompletely, which reduces volumetric efficiency. It should be noted that VE is only a consideration at full throttle. At any other time, the VE is intentionally restricted by the throttle plate. So for the sake of this discussion, we are only dealing with full-throttle conditions.

So, in addition to the short time and intake restrictions that reduce the intake charge filling, similar problems exist on the exhaust stroke. Here, the piston is rising in the cylinder, the exhaust valve is opening, and the piston is pushing the exhaust out of the cylinder. Once again, physical restrictions to flow in the path the exhaust limit the amount of exhaust gases that can be cleared from the cylinder on the exhaust stroke. As a result, some residual amount of exhaust gas remains in the cylinder after the exhaust stroke. This gas takes up space during the intake stroke, further reducing the amount of intake charge gases that can enter the combustion chamber. All of these factors serve to reduce the VE, which reduces power.

Some of the factors that affect VE cannot be changed in an engine. The amount of time available to move gases in and out of a cylinder is dictated solely by engine speed. The obstructions to gas flow are inherent to the design of the intake manifold and cylinder heads. Of course, those can be changed and often are in order to increase VE. Larger valves, ported heads and manifolds, etc. all reduce restrictions to gas flow and thereby improve VE. So does camshaft selection. This is done using three different techniques – increasing valve lift, increasing valve opening duration and increasing valve overlap.

Lift is very simple to understand. The more a valve opens, the more gas that can pass through it. While there are subtleties that I won’t get into, in general, the more lift, the better. The only real drawbacks to high lift are the physical limitations of the engine (valve spring compressed height, piston-to-valve clearance, etc.) and the fact that the farther you open a valve, the more spring strength you need to close it without float. Duration is similar to lift in that the longer a valve remains open, the more gas it will flow.  Overlap is the amount of time that both valves are open, and the amount of overlap depends on duration as well as the lobe separation angle (LSA). The LSA is the number of degrees of separation between the centerlines of the intake and exhaust lobes. You can see how duration and LSA affect overlap in the diagram below.

In a typical street performance engine build with a single plane 4-barrel manifold, durations are typically more than 225 degrees (measured at 0.050 lift), with a relatively small LSA of something like 108 degrees. This produces a lot of overlap that has the effect of scavenging the exhaust gases. When both valves are open, the escaping exhaust gases help to draw the intake charge into the cylinder. This increases power at the top end at the cost of reduced power at the low end as well as poor idle. The intake valve is opening as the piston is still on its upward stroke, and at lower revs, the intake charge is not moving very fast so it has very little momentum. Additionally, the intake manifold is under vacuum. The upward motion of the piston creates a slight positive pressure by pushing some exhaust gases through the intake side that overcomes the momentum of the intake charge and gets sucked into the manifold by the vacuum. This is called “reversion”, and is why big cams have a “lopey” idle and don’t perform well at low revs.

Webers need a completely different approach to camshaft selection. First, because of the very short runner that provides the fuel charge a very direct route to the combustion chamber, there is much less resistance to flow, must less gas that needs to overcome inertia and improved VE. As a result, less camshaft duration is necessary to get the same result. More importantly, though, is that Webers are not very tolerant of reversion. In a large, shared-plenum manifold, a reversion pulse does disrupt the fuel charge flow, but that’s about it. The manifold is large enough to simply absorb the pulse for the most part. With the Weber IDA, though, each cylinder has its own very small runner leading to its own individual carburetor just a few inches away. The reversion pulse will blast back up through the carb, blowing fuel out of the top of the carb. Not only is this messy and a fire hazard, it completely disrupts the column of intake charge that was queued up to enter the engine. Just a short instant later, the flow must completely change direction. The inertia of the fuel charge is in the complete opposite direction and it takes time and energy to revers the flow back into the right direction. All of this serves to reduce VE. Additionally, whereas with a shared-plenum manifold the effects of reversion tend to reduce with RPM, that’s not necessarily so with Webers on independent manifold runners. You can see the effects of reversion in this video. Notice the fog of fuel that forms outside the mouth of the carbs:


Those are Weber DCOE carbs, which are essentially IDAs configured as side-draft carbs. The end result is the same. Weber carbs on independent runners do not like reversion. To minimize reversion, you have to minimize valve overlap. This can be done in two ways - reduce duration, or increase LSA, or both. How much you do of each will depend a lot on your performance goals. One of the aspects of Webers that make them desirable is their ability to provide a lot of torque particularly low in the RPM range. My approach is to build upon that strength and select a cam that will favor operation in the low to mid-upper range. People brag about horsepower, but torque is what wins races. Horsepower peaks at the top end, but torque comes in early and gives you that kick in the pants upon acceleration. Race car drivers will tell you that horsepower is for bragging, but torque wins races. It’s what pulls you out of the exit of a corner and squirts you to the next one. To optimize torque with Webers, you want as much lift as your mechanical systems will handle, comparatively short durations and relatively high LSA. The camshaft that I selected was manufactured specifically for Webers and is available through the Inglese division of Comp. It is the Stage 2 cam for Webers on the small-block Ford and has the following specs:

Lift: I-0.571/E-0.565

Duration: I-222*/E-224* (at 0.050” lift)

LSA: 115*

You’ll notice that the lift is quite large, duration numbers are low to middle-of-the-road for a performance camshaft and the LSA is quite high compared to the more typical 108 - 110 degrees. The modest durations and large LSA will minimize overlap, and combined with the large lift, will maximize the vacuum signal, and everything works together to optimize torque throughout most of the range while not sacrificing too much top-end. I am currently running a 6500 RPM redline and the engine pulls very nicely all the way to redline.

In summary, if you have the option to select the camshaft, choose one with a lot of lift, modest duration and a LSA of 114* or greater.


Webers like timing - lots of it. They also like the timing to come in early. I have ready many accounts of people trying to tune out a stubborn problem by changing jets, holders and air correctors, only to find out that a few more degrees of timing advance made the problem go away. Don’t make the mistake of using too little timing advance. I know that conventional wisdom is that too much advance can lead to dangerous detonation, and this still holds true for total advance, but you might be surprised how much initial advance Webers can tolerate. I have found that 18 degrees of initial advance works well for me. I have the distributor curved to bring another 20 degrees of advance in by 2000 rpm for a total advance of 38 degrees. The total timing remains the same as “stock” but the difference is the amount of initial advance and how quickly the total is brought in.

Fuel Pressure and Supply:

Another finicky aspect of Webers is the fuel pressure they will tolerate. Whereas most carbs are perfectly happy with 7psi or so of fuel pressure, Weber IDAs don’t like anything greater than about 3psi. I have read many accounts of people having all kinds of difficulty with their IDAs being intractable, fuel-spitting monsters only to find that they were unaware of this particular quirk. If you run more than 3psi of fuel pressure to IDAs, you run the risk of overpowering the float valve and having the fuel overflow the float bowl. I keep my fuel pressure at 2.5 psi and that works great.

Another important consideration is the fuel supply. In particular, I’m referring to fuel temperature and the method by which it is delivered to the carbs. In most carb applications, the carbs are deadheaded in the fuel circuit. That is, the fuel pump delivers fuel to the regulator, which then delivers a regulated fuel supply to the carbs, where the fuel system terminated or deadheads. Here’s the problem. Because Webers sit in very short intake runners, they are very close to the hot heads. That, combined with modern fuels designed for return-style fuel injection systems that contain alcohol additives with a resultant low boiling point, leads to a common problem of fuel boilover in Webers. This is particularly troublesome when a hot engine is shut off. The carbs continue to heat soak, and any fuel that is left in the carbs will have a tendency to boil over into the carb bore and then leak though the bearings onto the manifold. Not only is this a fire hazard, but it will also dump fuel into the engine, leading to the potential for cylinder washdown.

One way to combat fuel boilover is to keep a fresh supply of cool fuel running to the carbs constantly. This is done by using a return-style fuel system - the same way that a fuel-injection system does. The key here is to put the carbs in the flow loop, rather than being deadheaded. Most Weber IDA systems today use a fuel loop that hits each carb in series. If you plumb the fuel system so that fuel is constantly flowing in this loop, it will keep cool gas in the carbs at all times. It should look something like this:

Fuel Pump-->Fuel Loop-->Regulator-->Return Loop

You can see how this arrangement looks in this pic:

In my installation, I have an EFI-style in-tank fuel pump that delivers fuel to the line you see at left in the picture just next to the passenger footbox. The fuel then goes directly to the fuel loop that loops around to each carb. The fuel line then goes to the regulator on the firewall, which regulates the pressure and sends the spill back to the tank through the line just underneath the supply line

At first, you might think that the fuel loop would not be at regulated pressure since it is before the regulator in the system. However, a return-style regulator regulates pressure by bleeding off fuel until the pressure setpoint is reached. So the regulator situated downstream of the fuel loop keeps the pressure in the loop at the setpoint by flowing enough fuel in the return line to keep the pressure bled down. It may seem counterintuitive, but it works very well.

Another advantage of using an electric fuel pump is that you can switch it off. This helps to deal with the hot engine shutdown problem. When you shut down a hot engine, you stop the flow of cool fuel to the carbs and what’s left inside them may still eventually heat up from residual heat in the engine and boil over. If you have a switchable fuel pump, you can turn the fuel pump off a quarter mile or so before reaching your destination to drain the fuel bowls before you turn the engine off. I find that this works very well. Of course, if you’re concerned about running our of fuel before you reach your destination, you can achieve the same effect by shutting off the fuel pump when you get there, and letting the engine idle for a minute or two to burn off the fuel in the bowls.

As long as we’re talking about fuel boilover, it is also worth mentioning that there are spacers available made of phenolic material of various thicknesses that are designed to insulate the carbs from the heat of the engine. I haven’t used them but I understand that they work marginally well. What I have tried is doubling the thick Weber carb-to-manifold gaskets, and that seems to help. That, combined with the cool fuel loop and the switchable fuel pump have completely eliminated boilover for me.

Float Setting:

Before doing anything regarding tuning of Weber IDA carbs, there are some steps that absolutely must be performed if you are to have any chance of success. The first is to set the float level. Each IDA carb has a central float bowl that serves both barrels. For reasons that will become clear in the next section, the level of fuel in the bowl is critical to proper operation of the carbs. Sadly, Weber does not set the float level at the factory, and unless you buy your carbs from someone who does this service for you, you absolutely must do this yourself. It is not a particularly complicated procedure, though it does require at least partial disassembly of the carbs. This is not a bad thing – it will give you a chance to get to know them.

There are a number of web sites that describe this process, but they can be a bit difficult to follow. So, I’ll describe how I did it in a bit of detail. Let’s start by looking at a diagram of the carb:

You can see that the float (15) sits in the bowl between the carbs. There is a little tab near the pivot point for the bowl that actuates the needle valve (12) when the float rises, thereby cutting off the fuel supply when the bowl is full. To understand why the float level is so important, look at the bores adjacent to the float bowl that appear to have bolts poking out of them. Those “bolts” are the jet assemblies. There is the idle jet assembly (49 & 50) and the main jet assembly (45, 46, 47 &48). These assemblies contain the jets on the bottom with the remaining parts of the assembly being hollow components with small holes in them that allow some air into the fuel to help emulsify the fuel. The bores that the assemblies sit in are open on the bottom to the float bowl. This means that the fuel level in the bores will be the same as the level in the bowl. The amount of air metered by the assemblies is directly related to how high the fuel is on the assemblies, and as a result, the fuel level can directly affect the fuel mixture. So, getting it right is very important. Unfortunately, it is not a simple matter to set the level. You need some special tools to do it properly. Fear not, though. They are fairly easy to make. Here is the diagram Weber uses to describe the process:

There are three tools needed. These used to be available from Weber, but no longer are. Reproductions are available, but they are actually easy to make. The first one is the gauge marked 98015.500 in the left-hand diagram. This is a gauge to measure the uncompressed height of the ball on the needle valve assembly above the deck of the top part of the carburetor. This critical measurement should be 25mm. The next tool is the one colored yellow in the middle diagram (98014.100). This is used to measure the float height above the bowl deck when in “measuring position”. This measurement is supposed to be between 5.5mm and 6mm. The tool depicted above has a 5.5mm measurement on one side and a 6mm measurement on the other. This “measuring position” is determined by the final special tool colored red in the right diagram (98014.200). This is a tool that bolts to the bottom half of the carb when the top half is removed. It take the place of the needle valve and its dimension - 24.2mm represents the compressed or closed height of the needle valve. Hopefully this will become clearer in a moment.

Here’s why all of these special tools are necessary. If you refer to the exploded diagram again, you will see that the needle valve attaches to the top section of the carburetor, whereas the float attaches to the bottom section. There is no way to see the level on the float bowl without taking the top section off, but if you take the top section off, you can’t set the fuel level because the needle valve comes with the top section. So, the fuel level has to be set indirectly by measuring the components, not by directly measuring the fuel level. So, let’s start with the first tool (98015.500). As has already been mentioned, this tool measures the height of the needle valve over the underside of the top section of the carb. This is very important because we will use a surrogate (tool 98014.200) for the needle valve when we measure the actual float height. So, the needle valve height must directly correspond to the length of the surrogate, or 25mm above the deck of the top section. So, I fashioned a tool out of sheet aluminum with a gap exactly 25mm and checked the measurement of each needle valve, being careful not to depress the ball:

So, what do you do if the height isn’t correct? Well, it’s all in the gaskets. There is a little fiber gasket under the needle valve and its thickness can be adjusted. If the needle valve is too high, you can lightly sand the gasket thinner using some fine sandpaper. If the valve is too low, you can double up on the gasket and sand them to bring them to the right height.

Once the needle valve height is perfect on all 4 carbs, it’s time to turn to tool number 3 (98014.200). This is the surrogate for the needle valve, and I made one out a piece of aluminum and a machine screw:

It screws into the carb body using the carbtop screws and acts like the needle valve by limiting the travel of the float. The tool is carefully set to 24.2mm (ignore the 24.0 written on the tool).

Here’s where I differ from the conventional wisdom. Weber says to use another tool (98013.800), which is essentially a small piece of bent sprint steel, to hold the float up against the needle valve surrogate tool. However, I found it much easier to just turn the carb base upside down.

Needless to say, this needs to be done with the carbs off of the car. You can see that the float projects above the base deck by some amount. This is where the other tool comes into play. It is simply a gauge that checks this projection. It should be between 5.5mm and 6mm. I made a gauge that is 5.75mm (right).

Make sure that your gauge has a notch that accommodates the seam of the float. You want to measure to the body of the float, not the seam, hence the notch.

The picture above shows how to position the gauge, but as I already mentioned, it is easiest to make the measurement by holding the carb body upside down and letting gravity position the float for you. If the float height is not right, simply adjust it by carefully bending the tab with some needle-nose pliers. Make very tiny adjustments and take your time and you’ll get there. The one consolation in all of this is that once you do this, you should never have to do it again.


In order for Webers to work properly, they must be synchronized. That means that for each carb, the throttle plate is opened precisely the same amount for any given throttle setting. To do this properly, you need a device called a synchrometer.

All this device does is measure air flow into the carb. You place it over the carb opening and read the air flow. The idea, of course, being to make each carb read the same.


Theory of Operation:

At the risk of getting too technical and scientific, to understand how Webers work, it is critical to understand some of the theory behind how a carburetor – any carburetor – works. The concept actually ends up being quite simple, even though the math and science behind the concept can be exceedingly complex. All carburetors work by utilizing differences in air pressure to cause fuel to move into and mix with the stream of air going into the engine. That deceptively simple concept is the fundamental principle of operation for all carburetors everywhere. It is true that some carburetors have features like accelerator pumps that mechanically pump fuel into the air stream, but those are temporary “fixes” that make up for shortcomings in the air pressure difference principle. Under steady-state conditions, all carburetors use a difference in air pressure to introduce fuel into the engine. I’ll get into the two primary methods by which this happens shortly. Before I do that, however, it is also important to discuss fuel mixture.

Gasoline does not burn. Well, it actually does burn, but only in vapor form. Liquid gasoline cannot readily combine with oxygen in the air to cause combustion. So, to burn gasoline, the gas needs to be a vapor and it needs some quantity of air to facilitate combustion. If the quantities of gasoline vapor and air are precisely metered to create a mixture such that every gasoline molecule is combined with an oxygen molecule in the combustion process and there is none of either left over, that mixture is said to be “stoichiometric”. Anyone who has taken basic chemistry remembers the subject “stoichiometry” where you developed and balanced chemical equations such as:

2H2 + O2 --> 2H2O

This is the simple equation for creating water from the combination of hydrogen and oxygen. In order for the reaction to be stoichiometric, there must be two hydrogen atoms for every oxygen atom. If that is the case, then each hydrogen and oxygen atom is consumed in the reaction and all that is left is water. Gasoline works in a similar fashion, though it is not quite so simple. Gasoline is not a compound, it is a mixture of compounds, and that mixture can vary from brand to brand, from grade to grade and from season to season. Additionally, gasoline engines don’t mix gasoline with oxygen, they mix gasoline with air, which is only 20.1% oxygen. So, a simple chemical equation like the one above for water can’t be expressed for gasoline and air. However, very smart people have already done the math, and the stoichiometric ratio for air and gasoline is approximately 14.7 to 1 (by mass). That means that every gram of gasoline introduced to an engine requires 14.7 grams of air in order for the combustion to be complete and for all of the gasoline and oxygen to be consumed equally. So, the ideal air-to-fuel ratio (AFR) for a gasoline engine is 14.7 to 1. If there is too much fuel in the mixture (AFR less than 14.7:1) the mixture is said to be rich. If there is not enough fuel (AFR greater than 14.7:1) then the mixture is said to be lean.

Sounds simple, right? Well, it seems that things are seldom as simple as they sound at first. There is a problem with a purely stoichiometric mixture. While it is the most efficient use of gasoline, it burns very hot. That’s not really a problem at idle and low engine speeds. In fact, modern fuel injected cars use oxygen sensors and a computer to adjust the mixture on the fly to maintain a stoichiometric ratio at low engine speeds in order to offer the best fuel economy possible. However, at higher speeds, the very high temperatures created by a stoichiometric mixture can cause detonation, which can quickly destroy engines. The solution to this dilemma is to cool down the combustion process. How is that done? Well, anyone who has spilled gasoline on their skin and felt the obvious cooling created by the evaporation will understand the mechanism. The idea is to introduce extra fuel into the mixture. The tiny droplets of fuel evaporate in the combustion chamber and that evaporation cools the fuel/air charge enough to prevent (hopefully) detonation. The degree of richness required depends on many factors, but for a performance engine at WOT, the ideal mixture is often between 12:1 and 13.5:1. If the mixture is too rich, power and economy suffer, and if it is too lean, detonation becomes a greater risk. Of course, some of the gasoline in this rich mixture is not burned because the mixture is not stoichiometric. So it just goes out with the exhaust. In fact, the primary function of the catalytic converter in modern cars is to complete the combustion of this unburned gasoline through a process called catalysis. It is worth noting that ignition timing also plays a crucial role in producing the best performance possible from an engine, but this discussion is about AFR only.

OK, so back to carburetors. We have now established that the carburetor must supply the engine with a mixture of air and fuel that is somewhere between 12:1 and 15:1, and that mixture needs to change through the RPM range. The mixture typically needs to be leaner at lower engine speeds and richer at higher engine speeds and this must be managed by the carburetors. How do they do this?

Let’s go back to the beginning. We have already established that carburetors deliver fuel by utilizing differences in air pressure, though we haven’t really discussed how. We’ll get to that in a moment. We also need to understand that carburetors also manage the delivery of air to the engine. How this works is critical to understanding how the fuel is delivered.

Carburetors don’t actually “supply” air to an engine. The engine does the supplying by creating a vacuum during the intake stroke. As the piston travels downward, and the intake valve is open, air is sucked through the carburetor into the cylinder. The carburetor manages the delivery of air by using a throttle plate. When the throttle is closed, the throttle plate blocks off the carburetor bore and very little air gets into the engine. As a result, the carburetor has to deliver very little fuel. It is very important to understand what is going on in the intake manifold under these conditions. The intake stroke creates a partial vacuum and with the throttle plate closed, not enough air gets into the engine to fill that vacuum, so a strong vacuum exists within the intake manifold when the throttle opening is small. This vacuum – a difference in pressure between the ambient air and the area under the throttle plate – is used by Webers to deliver fuel into the air. The fuel is simply sucked by the vacuum through very tiny holes into the air stream. In summary, at small throttle openings, there is very little air movement and a lot of vacuum under the throttle plate, and that vacuum is used to suck fuel into the air. As I said earlier, it is a very simple concept.

Things get a bit more complicated when the throttle plate is opened. Let’s consider the opposite end of the spectrum – full throttle. Now, the throttle plate is fully open and there is little impediment to the air rushing in to fill the vacuum. So, the air very quickly fills the manifold and cylinder and the strong vacuum we had at small throttle openings is gone. Instead, we have a very strong rush of air that did not exist before. Since the vacuum is now gone, there is no more ability to use static vacuum to suck fuel through the little holes into the air stream. A different mechanism must be used to deliver fuel. The mechanism used takes advantage of the rush of fast moving air that did not exist at small throttle openings. Enter the Bernoulli Principle. Anyone who ever blew across the top of a straw in a glass of soda as a child and saw the level of soda rise in the straw has seen this principle at work (if you’re never done this, try it…). Stated simply, the Bernoulli principle hold that a moving fluid (air is a fluid) exerts decreasing pressure perpendicular to the direction of flow, and the decrease in pressure is proportional to the speed of the fluid. So, when you blow across the top of a straw in a glass of soda, the moving fluid (your breath) causes a decrease in pressure at the top of the straw. It may look like the soda is being “sucked” up the straw, but in actuality, the air pressure at the top of the straw is now lower than the ambient air pressure on the surface of the soda in the glass. The ambient air pressure then pushes the soda up the straw. If you blow hard enough, you will draw the soda all the way to the top of the straw and even blow some of the liquid out of it. This is precisely what a carburetor does at large throttle openings. The fast-moving air lowers the pressure over the opening of a “straw” that is inserted into the air stream, and gasoline is drawn up the straw and into the air stream. This effect is enhanced by the “Venturi Principle”. In essence, all a venturi does is narrow the path for the air, thereby increasing its speed and further increasing the Bernoulli effect. All carburetors utilize a venturi around the “straw” where the gasoline comes out.

So, the bottom line is this. Weber carburetors operate in two different modes. The first mode is with small throttle openings. In this mode there is very little air flow and very high manifold vacuum. Fuel is delivered by using manifold vacuum to suck fuel into the air stream. The other mode is with large throttle openings. In this mode, there is a lot of air flow but very little manifold vacuum. Fuel is delivered by using the fast-moving air to suck fuel into the air stream using the Bernoulli principle.

Sounds simple, right? Well, we should all know by now that things are never as simple as they may seem at first. Life with carburetors would be grand if all we had to worry about was idle or WOT. In fact, the original Weber IDA carbs were designed primarily for these two conditions only. Since they were originally intended for racing, they were designed to function beautifully at large throttle openings with only passing attention paid to the rest of the throttle range. For street use, however, we want our engines to idle nicely, and to perform well throughout the throttle range – not just at small and large throttle openings. Weber IDAs can be made to do this, but it can be tedious getting them to do so.

It is worth pointing out at this point that Webers have a certain mystique that is both good and bad. They are renowned for their performance, but are equally known for their stubbornness and intractability regarding tuning. There is a pervasive sentiment that only those mystical shamans who have the magic Weber touch can get them to behave. Well, that simply isn’t true. Webers are machines that operate according to well-established scientific principles, most of which have just been discussed. Getting them tuned properly is simply a matter of understanding and applying these scientific principles properly.

How Weber IDAs Work:

There are many resources for tuning Webers. There are good books, many internet resources and a few mystical shamans scattered around the country. A list of good books is at the end of this article. However, most of those resources are very general in nature and address tuning all Weber carbs. I intend to focus on the IDA carbs only – a model that most of the books I have read address only in general terms. While the principles discussed in the books apply to the IDAs, the specifics of tuning these particular carbs are lacking. There are some qualities that are unique to IDAs and I will try to address those.

It is probably worthwhile to discuss what I mean by “tuning” the Webers. Essentially, it comes down to one primary objective. That objective is to get the carbs to deliver the appropriate fuel/air mixture throughout the performance envelope of the engine. To achieve that objective, Weber has graciously made the IDAs nearly infinitely tunable. This is both a blessing and a curse. The blessing is that for any application, there exists a combination of components that will make the carbs operate beautifully. The curse is that there are so many components, and the components’ effects overlap such that finding that perfect combination can be quite difficult.

So, with all this talk of components, it is probably time to introduce them. I have developed the chart below that identifies each “tunable” component. As this discussion progresses, and each component is discussed, the chart for each component will be completed. I hope that this approach will help the reader to better associate the component with its function and effects.


RPM Range Affected


Other Elements Affected





































As mentioned above, the table will be filled in as we progress through the discussion. The tuning discussion will focus on several ranges of operation for the carbs. These ranges correspond roughly to most of the components above, but there is overlap, and that will be discussed as well. I have selected my own terminology for these ranges partly because the more common terminology can lead to great confusion, or at least it did for me. The other reason is that these ranges are directly affected by tuning the corresponding components. The ranges are:

Idle range: This range is just as it sounds, although it isn’t really a “range”. It is at the very bottom of the engine speed range, and applies to idle only. It is in this region where manifold vacuum is highest and air movement through the carburetor bore is lowest.

Low-speed range: This range is from just off-idle up to somewhere around 2500 – 3000 RPM. In this range, manifold vacuum is still the primary means of fuel delivery, though at the top of this range, air movement is just beginning to start fuel delivery by the Bernoulli principle.

Transition range: This is a bit of an oddball range. It is where the low speed range and medium speed range overlap and manages fuel delivery by mechanical means to supplement any gaps produced by the transition from manifold vacuum fuel delivery and Bernoulli principle fuel delivery.

High-speed range: In this range, fuel is being delivered predominantly by the Bernoulli principle, but there are adjustments being made through a process called emulsification.

So, without further ado, let’s move on.

Idle Range:

This seems like a good place to start. It would seem that idle is one of the simplest aspects of a carburetor and in most cases, that’s completely true. With Webers, on the other hand, it’s not quite that simple. The Weber IDA contains what is referred to as the “idle circuit”. The name is a bit deceiving, since this circuit controls the mixture not only at idle, but throughout much of the normal driving range. For this section, we will discuss the idle circuit only as it pertains to actual idle.

Recall that at idle, there is high vacuum under the throttle plate and ambient air pressure just above the throttle plate. Also recall that there is very little air movement. At idle, the throttle plate is only very slightly cracked open, and the air that enters the engine does so around the edges of the throttle plate. Because the air must rush over a sharp edge through a small opening, it emerges on the other side being very turbulent. The amount of air that is allowed to enter is controlled by how much the throttle plate is cracked open. This is managed by a mechanical screw and stop system. Run the screw in on the stop and the throttle plate opens more. That’s how air, and ultimately idle speed is regulated by the IDA at idle.

In the IDA, there is an idle fuel orifice in the carburetor bore just below the throttle plate. Remember that any region below the throttle plate at idle sees the full manifold vacuum. Since the idle orifice is below the plate, the manifold vacuum pulls fuel into the turbulent air, and the two mix. The amount of fuel that can be pulled into the air through the orifice is regulated by a needle valve. Screw the needle valve in, and the mixture gets lean. Screw it out and it richens the mixture. Each barrel of the IDA has an idle mixture screw. That’s how fuel is regulated by the IDA at idle.

So, the only adjustments for idle are the throttle stop setting, and the idle mixture setting. There are no jets or holders or correctors, etc. that need to be changed to get proper idle. Having said that, actual idling is only part of function of the idle circuit. As ever, Webers tend to make things more complicated, as the next section will demonstrate.

Tuning the idle is perhaps the simplest task in tuning Weber IDAs, but even the simplest task can get complicated. The first step is to make sure that the carbs are synchronized. This is discussed in a separate section. The next step is to establish “lean best idle”. This can fairly easily be done by ear, though it may take some practice. The idea is to lean the mixture for each cylinder, one at a time, until that cylinder falls out. Even on a V8, this is fairly easy to detect if you listen closely for it. Turn the idle mixture needle valve in very slowly until you hear that cylinder start to stumble. Then very slowly back the screw out just until the cylinder comes back, then back it out about another 1/16 turn. We’re trying to get as close to a stoichiometric ratio here as possible, which occurs just before lean stumble, and then we want to richen it slightly. Here’s where things get complicated. When you adjust the idle mixture, it will likely raise the idle speed as you find lean best idle. The temptation would be to crank the idle speed down a bit with the idle stop adjustment and call it done, but what happens then is that you throttle down the amount of air being permitted into the carb after you have adjusted the mixture for lean best idle. The amount of fuel delivered is mostly dependent upon manifold vacuum, not air flow, so you have now made the mixture rich again. At this point, it is a good practice to repeat the process of finding lean best idle again, and adding a 1/16 turn of richness. It is possible that you may need to repeat this more than once. The idea is to get just on the rich side of lean best idle at the idle speed you want. Why do we want to slightly on the rich side? The answer makes sense if you think about it. As you crack the throttle just off of idle there is a region where the engine is still being fed fuel by the idle orifice alone, just before the next mechanism takes over (see next section). The problem is that as you crack the throttle open, two things happen – first, you are letting more air in. Second, you are decreasing manifold vacuum. Recall that fuel delivery by way of the idle orifice is dependent almost solely on manifold vacuum and does not depend on air flow. So, you are adding air and reducing vacuum, all of which tends to lean out the mixture. So, in order to prevent a lean stumble when the throttle is barely cracked open, we adjust the idle mixture just a little bit rich.

In later sections, I will be using a wideband oxygen sensor to help with tuning. However, don’t make the mistake of trying to tune idle with one, particularly if you have a fairly aggressive camshaft. The large overlap and late-closing intake valve will tend to dump raw fuel into the exhaust at idle and low speeds. This can cause an oxygen sensor to read artificially rich. The lean best idle method described above is far better. With a wild cam, even with a perfectly tuned idle, you will probably smell fuel. Just consider that part of the ambiance and mystique of the Weber IDA.

It is important to point out that we are only establishing a starting point for idle at this point. We may end up adjusting the idle mixture later to help tune the low speed range. We’ll discuss that in the next section. Now let’s update that chart:


RPM Range Affected


Other Elements Affected

Idle Adjust Screw

Idle, immediately off-idle and low speed

Clockwise - leaner.Counterclockwise - richer

Idle mixture also affects low speed mixture.


































Low-speed Range:

Low speeds (from idle up to 2500 – 3000 RPM or so) are regulated by the “idle circuit”. It is now time to describe this circuit in detail. This gets a bit complicated, so you may want to get a cup of coffee. Go ahead, I’ll wait…

OK. Now that we have caffeinated refreshments, let’s get on with describing the Weber idle circuit. Earlier, we hinted that the so-called “idle circuit” did more than just control the idle. As mentioned above, this circuit actually controls much of the normal driving range, from idle up to 2500 to 3000 rpm. As such, it is a very important range to get tuned right. We don’t do a whole lot of performance driving in this range, but for fuel economy and general streetability, this range is critical. After all, during typical street driving, this is where we tend to spend most of our time. So, let’s talk about how the rest of this circuit works.

As I’m sure everyone knows, a carburetor keeps a reserve of fuel inside a float bowl. The level in this bowl is controlled by a float attached to a needle valve. When the bowl is full, the buoyancy of the float causes the needle valve to close. In the IDA, there is a vertical idle chamber adjacent to the bowl that is connected to the bowl at the bottom. Therefore, the level in this chamber is the same as the level in the float bowl. Above the top of the fuel level in this chamber is a passage that leads to the carburetor bore. In this passage is the idle orifice - an opening from the passage to the bore that is controlled by the needle valve.

Let’s go back to idle for a moment. Recall that the idle orifice is below the throttle plate, and manifold vacuum is used to draw fuel through the orifice as controlled by the needle valve. This vacuum causes the fuel level in the vertical idle chamber to rise and fill the passage leading to the bore, and eventually draw the fuel out of the orifice. Simple enough.

However, in addition to the idle orifice, there are two (or three) other orifices drilled from the passage into the carb bore that are not controlled by a needle valve. These orifices are located progressively higher in the carb bore above the idle orifice. They are called progression holes, and they work in much the same way as the idle orifice.

As the throttle starts to open more, one edge of the throttle plate moves up in the bore, and the other edge moves down. The idle orifice and progression holes are located on the side of the bore where the throttle plate moves up. What happens next is the source of much confusion and understanding is critical to the correct operation of the Weber IDA. Remember that at low engine speeds, everything below the throttle plate experiences manifold vacuum, and everything above the plate experiences ambient air pressure. At idle, the idle orifice is below the plate so it experiences vacuum which draws fuel into the air stream. The progression holes are above the plate, so they experience ambient air pressure. As a result no fuel comes out of those holes at idle.

However, as the throttle continues to open, something magical happens. The edge of the throttle plate passes over the first progression hole, moving it from the ambient air pressure region above the plate to the vacuum region below. Suddenly that hole, that did not flow any fuel moments before, starts to flow fuel due to the manifold vacuum it is now experiencing. Of course, the idle orifice is still under the throttle plate, so it is still flowing fuel as well. As the throttle opens more, the next progression hole passes into the vacuum region and more fuel begins to flow. At this point, there are two progression holes and one idle orifice in the vacuum region flowing fuel. So, you can begin to see how the progression holes work. As the throttle plate opens, more air is allowed to pass, and the progression holes are moved to the vacuum region to supply the needed fuel to mix with the additional air.

You may have noticed that I said there were two or three progression holes. The original Weber IDAs had two progression holes. This made them very difficult to tune for the low speed range. Modern suppliers of IDAs drill a third progression hole to make the tuning easier.

So, now the low speed circuit is starting to make a bit of sense. It even may not seem all that complicated. Well, don’t get too comfortable. It may be time for another cup of coffee…

There are two problems with the progression hole arrangement. First, the more the throttle plate opens, the less turbulent the air is as it passes over the plate. This may not sound like a bad thing, but at low speeds, that turbulence is necessary to help the fuel mix with the air. Remember that liquid fuel doesn’t burn. The fuel needs to be vaporized, and the turbulence aids in that vaporization. Second, there must be a way to control the amount of fuel that can flow through the progression holes. Fortunately, the good people at Weber though of these issues and came up with an elegant solution that is remarkably adjustable.

Remember the vertical idle chamber discussed earlier that supplies fuel to the idle circuit? Well, that chamber is not empty. It contains two components called the idle jet and the idle jet holder. The chamber also has an opening to ambient air. The idle jet is much like any jet. It controls the amount of fuel flow that can enter the chamber. A larger jet allows more fuel to flow, and vice versa. That’s fairly straight-forward. The idle jet holder, however, is not quite so simple. As mentioned above, the idle chamber is open to ambient air through a small passage that connects the idle jet holder to the outside air. This causes something very interesting to happen. As the throttle opens and the idle chamber gets exposed to the manifold vacuum, not only is fuel drawn through the idle circuit, but air is drawn in as well. The air mixes with the fuel via holes in the idle jet holder, and the amount of air allowed is controlled by the idle jet holder in much the same way as a jet controls the amount of fuel that will flow. This air mixes, or emulsifies, with the fuel, giving it a head-start on vaporization. In fact, a similar effect takes place at idle. We noted earlier that the progression holes do not flow any fuel when they are above the throttle plate. However, since they are connected to the idle circuit passage and the idle orifice is under vacuum, they do flow air backwards into the passage. This helps to emulsify the idle orifice fuel to some degree.

That’s the nuts and bolts of how the idle circuit works. Now let’s take a look at the components and how changing them affects performance. The two components in the idle circuit are the idle jet (#49 in the diagram above) and the idle jet holder (#50 in the diagram above). The idle jet controls the flow of fuel, and the idle jet holder controls the flow of air for emulsification. As a first approximation, a larger idle jet will richen the mixture throughout the idle circuit range. However, the idle jet holder has a moderating effect on the idle jet. A larger idle jet holder will allow more air to flow, offsetting some of the fuel flow allowed by the jet. The amount of offset increases with throttle opening. These two components give a tremendous amount of flexibility that may not be obvious at first. For example, if the mixture is lean throughout the low speed range, a simple move to a larger jet may suffice. On the other hand, if the mixture is just right early, but leans out too soon, a smaller idle jet holder will decrease the amount of “buffering” of the idle jet and allow more fuel to flow later. Similarly, if the mixture is lean early but turns too rich later in the range, a larger jet and a larger jet holder may be needed. The bottom line is this – the idle jet and idle jet holder both affect the mixture, but the effect on mixture by the jet is more profound early in the range and the jet holder has a greater (though less dramatic) effect later in the range.

Finally, a note on initial idle mixture is in order. There are a lot of sources on the internet that make sweeping statements indicating that if you have to turn the idle mixture screw out more than one turn, the idle jet is too small. Other sources put this hard limit at 1-1/2 turns. Still others say that ¾ turn is the limit. So, what’s the right answer? In my opinion, none are right. First, idle mixture is determined almost completely by the idle mixture screw, and is only marginally dependent on jet size. I have changed out idle jets dozens of times and I almost never need to adjust lean best idle again. If I do, it is very close. The amount of fuel needed for correct idle mixture is dependent upon many things. A camshaft, for example, that is designed to give a very large vacuum signal at idle will draw more fuel at the same mixture setting as a “smaller” cam. The point I’m trying to make is that idle mixture alone is not a sufficient indication of idle jet size, or idle jet holder size for that matter. The correctness of these components can only be determined by observing mixture as the progression holes are opened. Having said that, it is important to remember that the idle orifice does more than supply fuel for idle. It continues to supply fuel as the progressions holes are exposed and even as the main circuit is active. So, the mixture at idle also affects the mixture during progression and at high speeds to a lesser degree. The rules of thumb listed above stem from observations that in general, if you have to add a lot of fuel to get the idle mixture right, the jet may be sized too small to be able to fuel the entire circuit. The correct combination of idle mixture, idle jet size and idle jet holder is such that a perfect mixture is supplied throughout the range of the circuit.

I’m also told by a trusted expert that over the years, the idle adjustment needle valve has changed at least 4 times, and that the different configurations would require a different number of turns. The point being that the proper tune for your car may or may not conform to what the internet says it should be.

Tuning the idle circuit can be a monumental challenge, or it can be a relatively simple and straight-forward process. The difference is whether you have the ability to see real-time feedback of the mixture in this range. I do much of my tuning with the aid of a wideband oxygen sensor. To me, this is the only way to be certain that you get this range right. If you’re going to tune Webers, do yourself a favor and install an oxygen sensor bung in the exhaust and invest in a good wideband sensor. I use the Innovate LC-1. It is relatively inexpensive and lets me watch or log the output on my laptop. Having said that, an oxygen sensor is not required and as pointed out above, it can lie to you. So throughout this tutorial, I will explain what I have done with the oxygen sensor as well as what you can do if you don’t have one.

The approach to tuning this range is simple - in theory. Slowly advance the engine speed while observing the mixture and then make changes as necessary. If you finished the previous tuning section successfully, you already have your idle properly tuned to just on the rich side of lean best idle.

You now want to take the car on the road. I like to do a slow pull in second gear and watch the AFR. From a rolling start at about 1000 - 1200 RPM, very slowly advance the throttle. You don’t want the accelerator pump to contribute significantly to the fuel ratio, so advance the throttle a bit, and then hold and check the AFR. Keep this up until you reach about 3000 RPM. If the mixture starts to lean out as you come off of idle and stays lean, you should immediately try a larger idle jet. Keep in mind that every time you change an idle jet, you may need to reset the idle mixture with the idle screws. If the mixture starts to richen as the circuit progresses, you may need to try a smaller idle jet. If the mixture looks good through the first part of the low speed range, but either falls off near the top, or goes rich near the top, you can try a different idle jet holder, with the larger holder tending to lean the top out and vice versa for the smaller holder. Bear in mind that the idle jet holder has a much less pronounced effect  than the idle jet does.

It is also important to bear in mind that an oxygen sensor can lie to you, as pointed out earlier. It is a tool that you can use to help with your tuning, but you should always rely on your senses and other cues as well.

If you don’t have a wideband oxygen sensor, you can still tune the low speed range. It just takes more time and some careful observation with your own sensors - your ears, nose and fingers. Rather than reading the AFR from a sensor, you’ll be listening carefully to what the engine is doing, sniffing for unburned fuel, etc. The idea is to detect stumbling and decide what is causing it. When you notice a stumble, you need to note the RPM and be alert to exhaust smells. If the stumble is accompanied by the scent of raw gasoline, it is likely, though not certain, that the stumble is due to a too-rich mixture. Of course, the only way to tell for sure is to change out a component to see if the stumble gets better or worse. This can be time-consuming and frustrating, but also rewarding.

If the engine stumbles, listen for pops through the carbs or the exhaust. Pops through the carbs generally (but not always) indicates a lean condition, and popping in the exhaust often (but not always) indicates a rich condition. You can also perform a plug cut to get a clue to how the engine is running in this range. Drive the fully-warmed car for about 10 minutes being careful to move the throttle as gently as possible to keep the impact from the accelerator pump to a minimum. The idea is to keep the engine in the low-speed range  while avoiding the other ranges. Once the ride is over, quickly shut the engine off, without letting it idle. Then pull some plugs and look at the color. A golden brown color indicates a good mixture. A black, sooty plug indicates a rich mixture, and a white dry color indicates a lean condition. This will give you an overall picture of the average AFR for this range.

For me, I started out with a 70 idle jet and a 120 idle jet holder. I found that my mixture went very rich just off-idle, and then transitioned to a lean condition near the top of the range. This indicated that the idle jet was too large, as was the idle jet holder. As an aside, it is generally not a good idea to change more than one component at a time. So even though I knew that my jet and holder were both too large, I persevered to adjust only one at a time. I tried a 65 idle jet. This improved the off-idle richness, though it was still present, but the higher speed leanness got much worse. So, I switched to a 100 idle jet holder. This made the higher speed lean condition improve, but it didn’t completely go away. After some more trial and error, I settled on a 60 idle jet and an 80 idle jet holder. Obviously, your results may be different, but this is what worked for me.

So, let’s update our chart:


RPM Range Affected


Other Elements Affected

Idle Adjust Screw

Idle, immediately off-idle and low speed

Clockwise - leaner

Counterclockwise - richer

Idle mixture also affects low speed mixture.

Idle Jet

Available sizes: 50, 55, 60, 65, 70, 75, 80

Mainly idle through 2500 - 3000 RPM but entire range is affected

Smaller - leaner throughout the range

Larger - richer throughout the range

Changing the idle jet means that you may need to re-adjust the idle mixture with the Idle Adjust Screws

Idle Jet Holder

Available sizes: 60, 70, 80 90,100, 120, 140

Mainly idle through 2500 - 3000 RPM but entire range is affected

Smaller - richer in the upper end of the range

Larger - leaner in the upper end of the range

The Idle Jet and Idle Jet Holder work together.

























A note about sizes for Weber components - in general, if the size is a number alone (such as a size 55 idle jet) the number represents hundredths of a millimeter. So, a size 55 idle jet has a orifice that is 0.55 mm in diameter. This convention does not hold for any designation that is accompanied by a letter. For example, with an F11 emulsion tube, the “11” means nothing other than the part designation. It is not related in any way to how the part is constructed.

It is also worth mentioning that the the change from one jet size to another is not always the same in terms of fuel flow. The area of the jet orifice determines the amount of fuel flow. For example, idle jets are available in size 50 through 80 in increments of 5. In going from a size 50 to a size 55, consider that the jet area for the size 50 jet is 0.196 mm2. For the size 55 jet, the area is 0.238 mm2. The size 55 jet is therefore 21.4% larger than the 50 jet and will therefore flow 21.4% more fuel. By comparison, the size 75 jet has an area of 0.442 mm2 and the 80 jet has an area of 0.503 mm2. The size 80 jet is 13.8% larger and will therefore flow 13.8% more fuel. As you can plainly see, the size divisions for the smaller jet sizes produce a more dramatic change than for the larger jet sizes.

As long as we’re talking about jets, it is a good idea to make sure that you buy only genuine Weber jets. I am told (but haven’t verified with Weber) that the genuine jets are flow-tested to ensure uniformity. I am also told that there are a lot of cheap Chinese jets on the market that are not flow-tested and may therefore not perform as well as the genuine parts. For this reason, it is not advisable to drill out jets to create different sizes. You may get the orifice sized correctly, but the more important flow rate may not be correct. Having said that, I have been known to drill out a set of jets that I have on hand to try to zero in on the correct jet size. Once I think I am close, I will buy the genuine Weber parts in the size I drilled out.

Transition Range:

It can certainly be anticipated that there is a point where a transition occurs between fuel delivery by manifold vacuum and fuel delivery by the Bernoulli principle. One might think that transition is what this section addresses. Well, one would be wrong – at least under typical circumstances. During smooth acceleration with well-tuned IDAs, the transition from the low-speed range with fuel fed by manifold vacuum to mid-speed range with fuel fed by the Bernoulli principle should happen seamlessly since the two circuits that manage the two ranges are designed to taper off and on respectively and to overlap to accommodate that taper. We’ll get to that type of transition in a moment. In this section, we’ll talk about a different type of transition – one where you romp on the go-pedal, opening the throttle plates suddenly from low speed. Before the above-referenced romping, the idle circuit is happily delivering fuel based on the suction created by the manifold vacuum. When the romping takes place, the throttle plates jump from mostly closed with high manifold vacuum and low air flow to wide open with low air flow. The open state of the throttle plate immediately kills all manifold vacuum so fuel delivery by that mechanism ceases. Meanwhile, the engine hasn’t spun up enough to create enough air flow to initiate adequate Bernoulli principle fuel delivery. As a result, the fuel system would instantly go almost completely lean and the engine would fall flat on its face if there wasn’t some other mechanism in place to provide fuel.

Fortunately, the folks at Weber thought of this scenario too. They engineered an accelerator pump that basically consists of a brass piston, actuated by the throttle linkage that pumps gas into a nozzle, or pump jet, that sprays the gas into the carb bore.

So, when you press the accelerator sharply, there is a mechanical spray of fuel to fill the gap until the engine catches up with enough air flow to get the Bernoulli principle going. The system is also designed such that if you slowly roll into the throttle, little or no fuel is sprayed by the pump jet. As you might imagine, this range is quite tunable as well. There are two primary tuning components – the pump jet and the pump bypass exhaust valve. The pump jet is just the spray nozzle that does the spraying. A large pump jet will spray a lot of gas over a short period of time. Conversely, a small jet will spray a smaller amount over a longer period of time. If that weren’t enough, there is also the pump exhaust valve. This is a relief valve of sorts that is plumbed in parallel with the pump jet. When the piston applies the pumping force, some of the gas will go through the jet and get sprayed, but some will be “wasted” through the exhaust valve and recirculated to the float bowl. This helps fine tune the pump spray.

Above is a pump exhaust valve (part # 11 in the diagram). Inside the valve is a steel ball bearing, and at the top and bottom of the valve is an opening. The valve sits in the float bowl just under the float tang. When the pump is not operating, the ball is at the bottom of the valve, allowing fuel to enter the piston chamber. When the accelerator is pressed very slowly, the piston does not offer enough pressure to lift the ball up into the closed position, and all of the gas will flow past the ball back into the bowl. On the other hand, when the accelerator is pressed sharply, the piston depresses quickly, forcing enough fuel into the valve to cause the ball to rise and cut off flow into the bowl, sealing off the escape route and thereby directing flow to the pump jet (#10 in the diagram). If you look at the valve closely, you will see a hole bored into the side. This is the exhaust orifice. It allows some of the fuel in the pump circuit to bleed back into the bowl, thereby reducing the overall pump shot. The valves are available with exhaust orifices in various sizes, including a valve with no orifice at all. That one directs all of the fuel to the pump jet.

The relationship between the pump jet and the pump exhaust valve is a complicated one. It may help to illustrate the relationship by looking at few hypothetical examples. For these examples, we will assume that the accelerator is pressed quickly enough to force the exhaust valve closed and that the volume of fuel produced by the piston always remains the same. This is a fair assumption since the actual piston displacement volume is not generally considered a tunable parameter, though there are ways that are beyond the scope of this discussion. Just for the sake of these hypothetical examples, we’ll assume that the piston will displace 1 ml of fuel with every pump and that it delivers that fuel at a constant pressure. That 1 ml of fuel will be delivered to the pump circuit and will be shared between the pump jet, which delivers the fuel to the engine, and the pump exhaust valve, which wastes the fuel back to the float bowl.

So, let’s look at the configuration that the IDAs have as they come from the factory. They are equipped with a size 50 pump jet (with an orifice of 0.5 mm in diameter) and a pump exhaust valve in size 00, which means that no fuel is exhausted. This means that the pump jet will deliver the full 1 ml of fuel to the engine. We’ll arbitrarily assume that this delivery takes 2 seconds for a fuel delivery rate of 0.5ml/sec. That is the fuel delivery rate of that pump jet. Now, let’s replace the pump jet with one that is twice as big. Before you reach for the size 100 jet, remember that the important aspect of a jet size is the area of the orifice. The jet size that most closely matches twice the area of the size 50 jet is a size 70 jet. Remember that area increases with the square of the diameter. So, the size 70 pump jet will flow twice the fuel as the size 50 jet. That means that the entire 1ml of fuel will be delivered in half the time, or 1 second.  The fuel delivery rate for the size 70 jet is therefore 1ml/sec.

Now, let’s look at the scenario where we go back to the size 50 pump jet and replace the size 00 pump exhaust valve with a size 50 valve (which also has a 0.5 mm orifice). Now, what we have effectively done is to split the pump volume into two equal paths. The pump jet will still deliver fuel at a rate of 0.5ml/sec, but half of the total fuel will be bled back to the bowl by the exhaust valve, resulting in only 0.5 ml being delivered to the engine over a period of one second. So the net effect is that the delivery rate for the size 50 pump jet remains the same, but the duration of the shot is cut in half by the exhaust valve.

If we modify the above scenario by inserting a pump exhaust valve that is half the size of the size 50 valve (size 35 is close), then we find that ¼ of the fuel is exhausted to the bowl, leaving ¾ ml for the pump jet, which, as we have already determined, can deliver fuel at a rate of 0.5ml/sec, resulting in a pump shot that is similar to the one above, but lasts 1.5 seconds instead of 1 second.

So, as a first approximation, the pump jet determines the flow rate of the fuel delivered to the engine and the pump exhaust valve influences the duration of the shot.

This relationship can be illustrated in the table below. You can clearly see how the pump jet and pump exhaust valve are related and the effect the pump exhaust valve has on the duration of the pump shot.

Keep in mind that these are all relative numbers based on the fictional scenario where the pump piston moves 1 ml of fuel and a size 50 jet will flow 1 ml/second. The actual values aren’t as important as the relationships of the numbers. The bottom line is this – the pump jet determined the rate at which the fuel is delivered, and the pump exhaust valve determines the duration of the pump shot.

Tuning this circuit can be a real challenge. Part of the reason has to do with the method of fuel delivery. All of the other methods involve at least some mechanism to help the fuel vaporize. Remember that liquid fuel does not burn, only gasoline vapor does. The idle and progression circuits use turbulence and emulsification to help the fuel turn to vapor before it reaches the combustion chamber. The pump circuit uses nothing. It shoots a squirtgun-like stream of fuel down the middle of the carb throat. If the engine is cold, much of the fuel will go unvaporized and therefore unburned. Even in a warm engine, much of the pump shot may go unburned because of lack of vaporization. It may seem counterintuitive, but too much fuel in the pump shot can actually result in a lean condition. That’s because liquid fuel does not contribute to the air-too-fuel ratio since it cannot burn. A smaller pump jet will produce a smaller stream of smaller droplets that will more readily vaporize in the fraction of second available before the fuel enters the combustion chamber. As a result, you could start out with what looks like a lean stumble in the range that is solved by going to a smaller jet.

Another influencing factor is the rate at which you roll into the throttle. If you snap the throttle open, you quickly close the pump exhaust valve. On the other hand, if you roll into the throttle slowly, the valve won’t close at all and the pump shot will be recirculated back to the fuel bowl. Of course, the more slowly you roll into the throttle, the more orderly the transition from the idle circuit to the mains, and therefore the less need there is for something to fill the gap. The goal, then, is to tune the pump circuit such that it is there when needed, but not there when it’s not needed.

Yet another factor to consider is the engine load when you stomp the go-pedal. If you’re in a low gear, the engine will rev very quickly, which will quickly start the air moving in the carb throats, thereby engaging the main circuit fairly quickly. In the other hand, if you romp on it in high gear, the engine will rev much more slowly, requiring a longer pump shot to tide things over until the mains kick in. Unfortunately, the pump circuit has no idea what the engine load is, and it will happily provide the identical shot in each case.

As you can probably surmise, is not possible to tune the pump circuit to be perfect under all conditions. This region requires compromises. Fortunately, the time an engine spends in this region is very short, so the engine is quite tolerant of the fuel mixture being wrong for a short time.

If you have an oxygen sensor, hook it up and take the car for a ride.  It is best to pick a repeatable condition under which you’ll test the pump circuit. I like to use a flat stretch of road in 2nd gear. Pick a starting speed that puts the revs somewhere around 2000 RPM, and then stomp on it. Pay close attention to what the engine does, and to the AFRs if you have the ability. With a perfectly tuned pump circuit, the engine’s response to the throttle should be almost instantaneous and there should be no stumbling or hesitation. If the engine instantly flops or hesitates, you should probably start with the pump jet. If you have an oxygen sensor, it will give you a hint as to which way you need to go (though remember that oxygen sensors can lie). If you don’t have an oxygen sensor, pay attention to things like popping. If you hear popping through the exhaust, it’s a hint that you are too rich. If you hear popping through the carbs, you may be too lean. Replace the pump jet with the next size rich or lean, and then repeat the exercise.  Remember, we are interested in that instant immediately following the pedal going to the floor. Keep experimenting with pump jets until you get the engine to instantly pick up when the accelerator is floored. Don’t worry at this point whether some stumbling follows. We just want the engine to instantly jump when romped.

Once you have a pump jet that causes the engine to instantly respond, you want to make sure that the shot duration is right. I find that the best way to do this is under the same conditions as above, but in 3rd gear instead of 2nd. This will place the engine under load for longer and will therefore test the pump exhaust valve more thoroughly. Ideally, the engine should still respond instantly when the throttle is pressed sharply, though it may stumble after the first half second or so of response. If you don’t get an instantaneous engine response – even a short-lived one – then you’re not done with the pump jet experimentation. Here again, an oxygen sensor can give you a hint which way you need to go. The idea is to build on the instant response you tuned above and make it last long enough for the mains to take over. So, if there is a stumble after the initial response, it is because the pump shot is either too short or too long. Once again, your ears may give you a clue if you don’t have an oxygen sensor. If you decide, or guess, that the pump shot is not long enough, try a smaller pump exhaust valve, and vice versa.

Once you feel like you have the 3rd gear conditions properly tuned, try 2nd gear again and make sure it still works well. This is often an iterative process as you find a combination that works for some conditions and not others. In any case, I recommend changing the pump jet first, and then modifying it by trying different pump exhaust jets as needed.

At this point, it is worth talking about a common problem with these carbs. It is not unusual to have a stubborn stumble in the 2000 to 3000 RPM range that can be very difficult to tune out. If you think about it, it is really not surprising. There is a lot happening in that range, particularly when under moderate to hard acceleration. The idle jet flow is regulated by the idle jet and is heavily influenced by the idle jet holder. The main system is just beginning to come on line, and the pump jet, along with the pump exhaust valve, is being asked to fill in the difference. There are 5 components that need to be just right for this range to work right - the idle jet, idle jet holder, pump jet, pump exhaust valve and main jet. Those are just the issues associated with carburetion. Timing is also undergoing changes in this range. This is where mechanical advance starts to come in, and with Webers, timing can play a big role in performance. Not only does the base timing need to be right, but also the amount of mechanical advance and the advance curve also need to be just right. Timing, therefore, adds three more adjustment possibilities to the mix in this range. So, in reality, there are 8 adjustments that all need to be just right for this range to work. It is no small wonder that a lot of people have trouble with this region. The approach I took was to set the timing where experience with Webers says it should be and then make sure that fueling was right first. The oxygen sensor told me what I needed to do with fueling, and once I got that were I wanted, the stumble was almost gone, but not quite. I then started working on timing. I started with the initial timing at 18 degrees with another 16 degrees coming full-in at 3000 RPM. I ended up with 20 degrees coming in at 2500 RPM. That fixed the stumble completely. As I said earlier, Webers like a lot of timing coming in early.

I don’t have any perfect advice for getting this range right other than to say that it is worth it to concentrate on the things that have the most influence. When being gentle with the throttle, this region is dominated by the idle jet and idle jet holder. When being aggressive with the throttle, the pump jet and pump exhaust valve play a significant role. The main jet (discussed in the next section) is just starting to come in, and as such plays a lesser, though not insignificant, role. In my case, the biggest problem seemed to be the idle jet holder. I started out too big and was running lean in this region even though I was pig-rich in the lower RPMs. A smaller idle jet and smaller idle jet holder smoothed things out quite a bit. This put the mixture just right at low RPMs and allowed that mixture to remain correct until the mains picked up.

So, as you’re tuning this range, take heart in knowing that there is a combination of those 8 parameters that will work for you. You just need to find it. Remember to only change one parameter at a time, watch your AFRs closely if you have the ability, get your fueling right, and then fine-tune timing. So, now let’s look at that table:


RPM Range Affected


Other Elements Affected

Idle Adjust Screw

Idle, immediately off-idle and low speed

Clockwise - leaner

Counterclockwise - richer

Idle mixture also affects low speed mixture.

Idle Jet

Available sizes: 50, 55, 60, 65, 70, 75, 80

Mainly idle through 2500 - 3000 RPM but entire range is affected

Smaller - leaner throughout the range

Larger - richer throughout the range

Changing the idle jet means that you may need to re-adjust the idle mixture with the Idle Adjust Screws. The Idle circuit also contributes to the higher ranges so changes to this circuit will also affect mixture when the main circuit is active.

Idle Jet Holder

Available sizes: 60, 70, 80 90,100, 120, 140

Mainly idle through 2500 - 3000 RPM but entire range is affected

Smaller - richer in the upper end of the range

Larger - leaner in the upper end of the range

The Idle Jet and Idle Jet Holder work together.

Pump Jet

Available sizes: 40, 45, 50, 55, 60, 70

Sharp throttle movement at and RPM

Smaller -  shoots smaller amounts of fuel over a longer period of time

Larger - shoots larger amounts of fuel over a shorter period of time.

The Pump Jet works together with the idle circuit and main circuit to ensure a smooth transition from idle to mains

Pump Exhaust Valve

Available sizes: 00, 30, 40, 50, 60, 75, 100

Same as Pump Jet

Smaller - increases the overall volume of fuel available to the Pump Jet, causing a stronger spray

Larger - decreases the overall volume of fuel available to the Pump Jet, causing a weaker spray.

Same as Pump Jet.

















High-Speed Range

Once the idle circuit and pump circuit have completed their roles, and there is enough air flow though the carb throat to initiate the Bernoulli principle, the main circuit takes over. The lower part of the range of this circuit is influenced predominantly by the main jet, and the upper part of the range is more influenced by a component called the air corrector. The entire range of the circuit is influenced by the emulsion tube. So, let’s describe the components in this circuit in detail. Look at the figure below. Since I couldn’t find a similar drawing of an IDA, I will use this drawing of a DCOE (below), which is in large part no more than an IDA laying on its side. Most of the important components are very similar. Fuel enters the circuit from the fuel bowl through the main jet (5). The main jet meters the amount of fuel that is allowed to enter the circuit. The main jet is attached to the bottom of the emulsion tube (12) and the air corrector (11) sits on top of the emulsion tube. The function of the emulsion tube and air corrector can be a little difficult to grasp without a little more understanding of the physical principles that govern the operation of this circuit. There are two principles at work here – the Bernoulli principle, and the Venturi effect. Both are closely related and in fact, the Venturi effect is a corollary of the Bernoulli principle. Recall that the Bernoulli principle states that as a fluid’s velocity increases (air is a fluid) the pressure perpendicular to the direction of flow decreases. Remember the example that we used where we blow across the top of a straw in a glass of soda. If we blow quickly enough, the fast moving air over the top of the straw will cause a decrease in pressure in the straw, drawing the soda up into the straw. If we blow harder still, the soda will reach the top of the straw and blow out into the air stream. This is, in large part, what happens with the main circuit. Air rushing through the carb throat creates a low pressure in the fuel orifice that is in the middle of the air stream and the fuel is thereby drawn out into the air stream. The Bernoulli effect can be substantially enhanced if the throat of the carb is narrowed down somewhat and then opened up again. You can see this in the drawing below (9). This narrowing is called a venturi and it causes the air to speed up substantially, which causes the pressure to decrease accordingly, thereby multiplying the Bernoulli effect. Weber carbs also make use of a secondary venture called an auxiliary venturi (8). This is a separate venturi placed in the center of the throat and is the place where the fuel is actually delivered. The purpose of the auxiliary venturi is to further increase the velocity of the air stream, creating more vacuum on the fuel delivery circuit. It does this be locating the downstream side of the auxiliary venturi precisely where the lowest pressure is within the main venturi. This has the effect of sucking air through the auxiliary venturi and increasing the speed there. This, coupled with the venturi effect of the auxiliary venturi itself, creates a substantial low pressure area around the fuel nozzle (7).

So, that is how the fuel is delivered to the engine in the main circuit. But remember – liquid gasoline does not burn, only gasoline vapor does. You may recall that in the idle circuit, the idle jet holder had holes in it through which air was brought into the circuit to mix, or emulsify, with the air. The main circuit does the same thing though the use of the emulsion tube. Several varieties of emulsion tube are pictured below:

The main jet fits into the bottom of the emulsion tube, and the air corrector fits into the top, as pictured below:

If you look closely, you’ll notice that not all emulsion tubes are created equal. The different models have different middle sections that may vary the diameter of the tube and the size and location of the holes. To understand the meaning of these differences, let’s look at the diagram of the DCOE above once again. If you look closely, you’ll see that the whole main jet/emulsion tube/air corrector stack resides within a bore that is connected to the float bowl by a passage at the bottom of the stack under the main jet. This means that under normal circumstances, the fuel level in what I’ll call the main stack bore is the same height as in the float bowl.  The emulsion tube is open to fuel at the bottom by way of the main jet, and it is open to air at the top by way of the air corrector. So, as the throttle is opened and the main circuit starts to engage, the passage to the fuel nozzle (10) begins to drop in pressure. This drop in pressure draws fuel in through the main jet and air in through the air corrector. The fuel and the air meet in and around the emulsion tube.  The precise way in which the fuel and air interact is very important to understanding how this circuit works. To better help with that understanding, let’s look at a cross-section of a typical emulsion tube:

Remember that this tube resides in a bore that is only slightly larger than the tube. Fuel enters the tube through the main jet (not shown) at the bottom. It then flows though the fuel ports (G) and fills the bore.  The hollow upper part of the emulsion tube is an air passage and is open to the air above through the air corrector at the top (not shown) and it also gets filled with fuel up to the level of the fuel bowl.  This is illustrated in the (rather crude) drawing below. The big block on the right represents the fuel bowl, and the fuel nozzle is at the far left. As you can see, the fuel nozzle is slightly higher than the fuel level. The fuel will need to be “sucked” up the bore by the vacuum created in the venturi.

Here’s where things get complicated. As the throttle opens, the vacuum created by the venturi starts to draw fuel out of the main stack bore, but it also draws air in through the air corrector. The air enters the center part of the emulsion tube and exits through the holes, bubbling through the fuel. This bubbling emulsifies the fuel and air, creating a mixture that vaporizes very easily. This emulsified mixture travels to the fuel nozzle and is delivered to the engine.

At first, the fuel level will be near the top of the main stack bore, and the air will enter through the air corrector at the top of the emulsion tube and then exit the emulsion tube through the holes near the top of the emulsion tube. If the fuel level is above the level of the top holes, the air pressure will push the fuel down the bore inside of the emulsion tube until the holes are uncovered. At that point, the air will exit the holes , and bubble through the fuel trapped between the emulsion tubes and the sides of the main stack bore, emulsifying the fuel. As the throttle is opened more, the fuel that is in the space between the emulsion tube and the main stack bore walls gets depleted faster than the main jet can supply it. As a result, the fuel level in the main stack bore steadily drops.

Eventually, as engine speed increases, the fuel level in the main stack bore continues to drop, exposing more air holes, finally reaching the point where all of the holes are exposed and the fuel level is near the bottom of the bore:

So, there is a progression of sorts that takes place as the main circuit is engaged. At first, only a small amount of air is mixed with the fuel, since only a few of the holes in the emulsion tube are exposed. As the vacuum in the venturi increases, the fuel level around the emulsion tube drops, exposing more holes, thereby adding more air to the mix. You can now see how important the construction of the emulsion tube is. More holes mean more air, and holes at the top mean more air early whereas holes at the bottom mean more air later in the progression. The diameter of the emulsion tube also has an effect. It determines how much fuel the bore holds and as a result, influences how quickly the bore empties. The air corrector at the top of the emulsion tube is a jet of sorts that controls how much air is allowed into the holes. It tends to mostly influence the flow of air at higher vacuum levels in the venturi (i.e. at higher revs) since that is the region where it becomes restrictive.

All of these components work with the main jet, which regulates the amount of fuel entering the chamber. Not only that, it also regulates how quickly the chamber empties and finally, it regulates the amount of fuel delivered when the chamber is empty.


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