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Weber - Adjustment & Tuning Guide

In the coming months we will be adding a lot of information on Weber carbs, concentrating most of our efforts on the Progressive DGV, DFV, and DGS downdraft carbs. This article was complied from numerous articles and resources found on the internet, as well as our own experiences. The purpose of this guide is to cover the basics things you'll need to know to make your Weber carburetor run properly.  This is accomplished by setting the adjustments properly, and by re-jetting the carb so it meets the requirements of your specific engine. It's really pretty simple and not nearly as intimidating as you might think. For more detailed information, please refer to the “Weber Factory Tuning Manual” published by Haynes.

In a few weeks, we will be offering a tuning kit that will include several jet sizes, emulsion tubes, air bleeds and/or corrector jets. A deposit will be required, which is refundable when the kit is returned in good condition (minus the cost of any parts that are used, missing, or damaged). If you can afford to buy one, an A/F Ratio Gauge is an invaluable tool for tuning your carburetor, but it's generally too expensive ($500) for most customers, especially when you consider it's only use once or twice. For those that can't you need to learn how to read spark plugs, as this will get the job done.

The secret  to understanding the critical nature of the carb set up, and the advantages of a Weber over other carburetors, is the Idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the driving operation. This is the reason that the Weber should give an improvement in fuel economy over most factory carbs, along with significant performance gains. In a worst case scenario, you should'nt see a significant loss in fuel economy, but you should experience an improvement in power & driveability.

If your engine isn't running at it's peak potential, you may need to make a few adjustments to the carb. However there's a few things you should do before you start tearing into the carb. Remember.... a Weber carb is a performance upgrade, which was designed to deliver more fuel and air, and to make more HP over the stock carb. However... you need to make sure your fuel system supplies an adequate amount of fuel, at the proper pressure. You also need to make sure your ignition system is working properly, that the distributor was designed to work with a Weber carb (such as a Durasprk or DUI), and that it is capable of supplying an adequate spark to burn the extra fuel. Stock points style distributors are not up to the task, and need to be replace with a electronic distributor, an ignition box, and a hotter coil. Good plug wires are also required.

Vacuum Leaks
Let's back up just a bit..... before you go digging into your fuel system and the ignition, the first thing you need to do is check for vacuum leaks. Vacuum leaks are the #1 problem with carb swaps and conversions. This is particularly true of newer installs, as the chances of having a defective carb out of the box, are about the same as being struck by lightning. The first thing to check is the gaskets between the manifold, the adaptor, and the carb. In most cases we recommend using Silicone Gasket Maker between the manifold and the carb adaptor, since the adaptor never needs to be removed once it has been installed. This will greatly reduce the chances of a vacuum leak at that point. Next, we highly recommend a gasket between the adaptor and carb, so the carb can be easily removed for maintenance and tunning whenever needed. Make sure you use a new gasket, not an old one that may leak.

Fuel Delivery & Electric Fuel Pumps
Make sure that your fuel delivery is right. You cannot tune carbs that do not have adequate and proper fuel delivery, as it is critical to the proper performance of any Weber carb. From your basic 32/36 DGV to a set of triple side-draft carbs, all Weber carbs rely on a stable and full float bowl in order to mix the fuel and air correctly. Mechanical pumps very rarely do this. They pulse fuel instead of giving a smooth even delivery, and the amount of fuel varies with engine RPM.

A good electric fuel pump will provide the best performance and a stable supply of fuel for tuning your Weber carb. It should be noted that Weber carbs work best at 4 psi, not 2 psi, like many of the older books state. You also need a pump that will supply an adequate volume of fuel, not just pressure, as volume is what's needed to keep the float bowl full under periods of high demand. As such, we recommend a high volume, low pressure pump, that delivers 60-75 GPH at 4-5 psi.
The Carter 4070, which is a rotary pump, is an excellent choice. This pump does not need, and should not use, a fuel pressure regulator for any reason. Weber/Redline also makes a good fuel pump. In any case, make sure you use a high quality pump, rather than a cheap one.

Fuel pressure regulators are OK with stock fuel pumps that put out too much pressure, however a regulator is simply a restricter which may inhibit flow. This can result in a huge loss of volume, which may result in the float bowl going low or even empty under hard acceleration. Regulators
can also overload the fuel pump, causing a lot of noise and premature failure. Therefore, if you use a regulator with your stock pump, make sure it's a high quality regulator and that the fuel pump is supplying an adequate amount (volume) of fuel, and that it is capable of keeping the fuel bowl full at all times. We sell both Holley and Weber fuel regulators. The Weber regulator is twice the price, but it is highly recommended over the Holley regulator, as it was designed to work with Weber carbs. What ever you do, DO NOT use a High Pressure fuel pump, electric or mechanical.

Ignition System
First, you need to be certain that you have adequate spark to burn the extra fuel, other wise you may have problems tuning and jetting your carb. This is especially true with old cars that have point style ignition systems. A lack of spark, or weak spark, will make it impossible to tune the carbs properly, especially if your trying to achieve maximum performance. If you have a few extra bucks, we highly recommend replacing the old points style distributor with a good electronic ignition system, which includes a performance coil and plug wires. I guarantee you won't regret it.

NOTE: Ford distributors built from 64-67, called Load-O-Matic Distributors, didn't have centrifugal weights. Instead they relied on venturi vacuum to sense changes in RPM, which was supplied by a Spark Control Valve. The SCV was incorporated in all early stock Autolite 1100-1V carbs (until '68), which means when you replace the stock Autolite carb, you eliminate the Spark Control Valve, hence the Load-O-Matic distributor will no longer function properly. As such the distributor must be replaced when your doing a carb swap. Any late model or aftermarket distributor, that uses centrifugal weights to sense changes in RPM, will work fine. Click here for more detailed information.

Generally speaking, you may need a little more spark advance with a Weber carb. A good place to start for most cars, with Weber’s on pump gas, is 12-14 Degrees advance at 1000 RPM idle and 36 Degrees total advance by 3000 RPM...This is not cast in stone and you need to make sure your car does not detonate (or ping) at these setting. If you experience detonation under load, you'll need to back off (retard) the timing until it stops.

Always start with a fresh set of Spark Plugs. Do not clean them... start fresh. This will allow you to get a better plug reading, thereby enabling you to make a better judgment call on what needs to be done. If you have multiple carbs, you may want to do a compression test as well. If the results are off by more than 10% per cylinder you could have problems when you go to tune the carbs.

Factory Calibrations and Set-Up
Before you go tearing into your carb, the very first thing you need to do is make a record of the factory calibrations or set-up. In other words, what parts (sizes) were in the carb when you pulled it out of the box. Make certain you designate the differences, if any, between the primary and secondary circuits. Never rely on a list you found somewhere on a website or in a book, that says what your carb might or should have came with. Instead, remove the top of the carb and actually remove the jets, emulsion tubes, air correctors, check the sizes, then WRITE THEM DOWN. Don't use a small piece of scrap paper that could get tossed out in the trash, write them down inside the front cover of your Ford or Weber service manual, or any place where you'll have a permanent record that won't get lost. This way you'll always have the original factory settings if you ever need to refer or go back to them.

NOTE: Calibrations may vary due to regional fuel types and octane ratings. Poor running does not mean you have a defect in the carburetor. The number one and two reasons for tuning errors are improper linkage installations and binding from over tightened linkage.

Float Level - DGV
While you have the carb top plate off, this is a great time to check the float level. The float level is a critical part of carburetor calibration. Changing the fuel level in the bowl will change the point that the main circuit starts to feed. Float level also alters the characteristics of an emulsion tube, and affects the driveability and fuel consumption.

Float level, in mm, typically refers to the distance from the face of the carburetor top cover to the float. Start with the top cover held vertically (float pivot at the top) and with the float tab resting against, but not depressing the spring loaded ball in the needle valve. Measure the distance between the face of the carburetor top cover, to the top (or bottom) of the float. On carburetor models where it is required to remove the float to replace the top cover gasket, the measurement should be made with the gasket in place.

Choke Settings
All settings are done with the choke disengaged, or warmed up so that the choke is fully opened and disengaged. This is done on automatic choke carburetors by first opening the choke butterfly by hand and inserting a wood block, or a wedge of some kind. Next hold the choke open while the linkage is cycled (operated) through its full movement, to clear the choke cam. You will hear a metallic click when the cam is released. Visually check the fast idle screw, under the choke assembly, to confirm that it is not in contact with the chokes fast idle cam. Next, set the idle speed screw by backing out the screw until it is not in contact with the throttle stop lever. Cycle the linkage once more to be sure that the linkage closes without any assistance, while checking for linkage bind.  Now bring screw back into contact with the lever and continue to open, screwing in 1 turn but no more than 1½ turns.

Tuning the Weber Carburetor
Now that you have everything else sorted out, you can actually start tuning the carb. The good news is that you probably made the car run good enough by doing all these other things, that you may not need to do anything else. However, if your car is still not running right, or you feel you should have more power, then it is time to rejet and tune the carb.

NOTE: All adjustment procedures are the same for progressive and synchronous carbs, however it is important to understand the differences between the two carb styles. Progressive carbs idle through one barrel and one mixture screw, then transition to the secondary barrel which has an additional idle (low speed) jet. Synchronous carbs have individual idle jets and mixture screws for each barrel. They also have an additional air bleed screw and lock nut. The air bleed screw is not used for idle adjustment or quality. The settings for this screw should be closed. The adjustments settings for synchronous carbs tend to be half of what is recommended for progressive carbs. Example, if a progressive carb is set at 2 turns, the synchronous carb would be set at 1 turn.

First, you'll need to establish if you are running lean or rich? There is no way around this. You can be running rich at idle and lean on the main circuit, or vice versa. There are a few easy ways to establish this, and it is important to know, otherwise you won’t be able to cure the problem. The first option is to install A/F Gauge. It's fast and accurate, and it will take a lot of the guesswork out of the tuning process. The only drawback is that they are pretty expensive, but once you buy one, you'll always have it. Another option (the best one in our opinion) is to have your car tuned on a chassis dyno, by a professional tunner. This isn't cheap either, but the results will be well worth the cos, and it usually pays for itself over time, in fuel svaings. The final choice, and the most common, is to read the spark plugs. Ideally you want the ceramic part of the plug a nice Dark Tan to medium Brown color with a slightly darker ring right at the base of the plug threads. Remember, a new set of plugs may take a few minutes to get some color on them, so you'll need to run the motor for two to three minutes before shutting it down. The most important thing to know is white plugs are lean, while black plugs are rich.

Idle Mixture Screw and Idle Circuit
The idle mixture screw and idle circuit is CRITICAL to the overall driveability of the car. Not only does it control the idle, it also controls the entire low speed running circuit, as well as part throttle transition. The mixture screw is not just an air screw, as in other carbs, it actually opens a passage and lets in a pre-mixed volume of fuel and air. The more you open it, the more pre-mixed fuel enters the engine. Clockwise is leaner and counter-clockwise is richer.

Assuming you have no vacuum leaks this is a very simple process. However if you cannot get a good adjustment and you have to open the idle speed screw more than 2½ turns to get the engine to idle, then there’s a good chance you have a vacuum leak and you'll need to fix it before going on.

Set the mixture screw by first screwing it in until the screw stops, or bottoms out, then back out the screw 1½ turns. DO NOT FORCE IT, AS THIS WILL DAMAGE THE SCREW AND THE SEAT, WHICH IS IN THE BODY OF CARBURETOR. Start the engine and let it warm up. Do not worry about the idle speed at this point, just as long as the engine keeps running, as it will be re-adjusted later on. Start by turn the mixture screw(s) in, until the idle starts to stumble and get rough, then slowly turn them back out (turning ¼ to ½ turn at a time) until the best idle quality is achieved. Use your ear, not a scope or gauges, as you want to tune the engine by sound. The car should idle well and once your done, small adjustments clockwise (leaner) should make the idle speed drop off. Adjust to the best, fastest, and smoothest running point.

Low Speed Calibration
If the mixture screw is more than 2 1/2  turns out, then the idle jet is too lean (too small). When the mixture screw is less than 1 turn, then the idle jet is too rich (too large). These assumptions are based on the fact that the speed screw setting is not opened more than 2 turns. If the speed screw has to be opened 2 or more turns then this is also an indication of a lean condition usually requiring greater change. At times it may appear to be showing signs of richness or flooding, but it is really a lean condition. Please understand the need to keep throttle plate as near to closed as possible, so as not to prematurely expose the transition holes. This is what causes a visible rich condition, and confirms the need to increase the jet size.

The 38/38 DGES carbs can be a little tricky because you are idling on both barrels at the same time. You have 2 mixture screws, which WILL NOT be set the same on most carbs. This is because the plenum type log manifold distributes fuel unevenly. By having 2 mixture screws you are delivering fuel from 2 places in the intake manifold. Do not stagger the idle jets (in other words, do not use 2 different size idle jets) even if you need to adjust the screws differently, as this can cause part throttle driveability issues.

Idle Speed
Now that the mixture screw is at its best running location, you can adjust the idle speed screw. The screw will be sensitive and should only take ¼  to ½ turns to set the idle speed to your driving preference. Don’t set it too high, as this causes excessive wear on the clutch and brakes. The idle speed should be around 700-900 RPM. Recheck the speed screw and note how many total turns from initial contact. The final setting should be under 2½ full turns.

The Main Circuit
Okay... so now your car should idle correctly. This is a good time to recheck the timing and vacuum hook ups, making sure everything is right. If so, it’s time to get the main circuit tuned properly. First, do a test drive. It should come off of idle good, and transition to the main circuit smoothly. If it falls on its face and will not take any throttle, or if it runs better if you back off the throttle, then you could have a few different problems. Note: If you do not have enough spark advance, you could experience the same problems and it may have nothing to do with carb tuning.

If you have a flat spot or hesitation when you first take off, then it is likely that the idle circuit is too lean. If you know for sure that you have the idle circuit set properly, then the primary main jet is too small. If the car transitions fine, but hesitates or falls on it face when the secondary is opened, you'll need to work on the secondary main jet.

Drive the car above 2000-3000 RPM for a few minutes then shut the engine off before letting it idle, and check the plugs. Do not let it idle when you are checking spark plug color for the main jets. Keep checking the plugs, after driving it for a few minutes, and change the jets accordingly.

Note: The DGV, DFV, and DGS carbs, unlike the DCOE or IDA carbs, have fixed chokes (what we call venturi's) so choke size is not an issue. DCOE and IDA carbs have removable chokes (or venturi's) which can be changed out, thereby altering the cfm rating of the carb.

Plug Readings
When you check the plugs, the tip should be a hazy brown. If they are white, you need to run a cooler plug and/or richen the mixture. If they are dark brown or black, you need to run a hotter plug and/or lean the mixture. Dark black usually means the plug is too cold, while dark brown means the mixture is rich. Blackish brown means cold and rich. For more information, please see our tech article on "Spark Plugs Readings"

High Altitude Tuning

Tuning for High Altitude operation, above 4000 feet, requires slightly different settings and a wider range of replacement jets, for both the idle and main circuits, as well as larger air correctors.

(click on image)

32/36 DGV
38/38 DGS

Idle & Main Jet Sizes
Idle jets come in .5 increments (50,55,60,65,etc), a "50" idle jet has a .5mm fuel hole.
Main jets come in .5 increments (150,155,160,etc), a "150" main jet has 1.5mm hole. 
The bigger the jet number, the richer it is. The smaller the jet number, the leaner it is.

HINT: Do not go up or down more than 1 step at a time when tuning the idle circuit, 1 step in idle jet size can make a HUGE difference.
Increase the main jet size 1-2 steps at a time.

Ordering Jet Kits from Classic Inlines
Classic Inlines stocks the standard aftermarket jet kits which are sold by other suppliers. These kits were designed for mild performance motors using Weber 32/36 or 38/38 carbs, and are very reasonably priced. However with a total of 10 jets (4 main jets, 4 air corrector jets, and 2 idle jets), the kits are very limited and they may not include the jets you need for your application. As such, you basically have two choices.... buy the standard jet kit and hope it has what you need, or go online and buy a handful of individual jets, in which case your probably still guessing on the sizes you'll need. Either way, you probably won't have precisely the right size and you'll end up using one that's "close enough". The only other option is to buy every jet size available, or at least a wide range of jets. The only problem here, the jets cost more when they're purchased individually. For this reason Classic Inlines decided to put together two Custom Jet Kits, a small kit with 35 jets, and a large kit with 75 jets. If you were to buy the jets individually, these kits would cost in excess of $200/$500, respectively. However, we do things a little different. Basically you put down a refundable deposit when you order the kit, then when your done tuning your carb, you return the kit and you're only charged for the jets you used. Yes the kits are expensive up front, but you'll have every jet size you would possibly need to tune your carb "properly", and your money is refunded when the kit is returned. NOTE: These kits are only available to customers who purchased a Weber carb from Classic Inlines. Please call, or click here for more details.

Ordering Jet Kits from other suppliers
Some companies may ask for the following information when you order a Jet Kit:
a) Make and Model of car / truck
b) Engine Size
c) Modifications to engine
d) Use of vehicle
e) Altitude vehicle normally runs at

Additionally you'll need to know which style of idle jets you have. DGV and DFV carbs came equipped with 2 different idle jet styles. In some carbs, both the Primary and Secondary idle jets are the same size, while in other carbs the Primary idle jet is larger (see photos below). You'll need to know how your carb is equipped when you order parts.

Small Idle Jet
Large Idle Jet


VIP: Make a record of the factory calibrations or set-up. In other words, what parts (sizes) were in the carb when you pulled it out of the box. Make certain you designate the differences, if any, between the primary and secondary circuits. WRITE THEM DOWN. Don't use a small piece of scrap paper that could get tossed out in the trash, write them down inside the front cover of your Ford or Weber service manual, or any place where you'll have a permanent record that won't get lost. This way you'll always have the original factory settings if you ever need to refer or go back to them.

While the instructions above should get your car running properly, we highly recommend taking your car to a qualified Weber Technician for final tuning, or preferably, having it tuned on a chassis dyno. Tuning the motor on a chassis dyno is worth every penny, and you can rest easy, knowing the carb and ignition system have been tuned properly for maximum performance and mileage.

We had a customer in southern California install a Weber 32/36 on his newly built performance motor. The shop that built and installed the motor, also set the carb up. Now these were great guys with excellent mechanical skills, however they didn't have a lot of experience with Weber carbs (most mechanics don't). As a result, they messed with the carb for two weeks, trying to get it tuned properly. When the customer finally got his car back, it was running OK, but he wasn't happy with the overall performance. Especially when he took into consideration all the money he spent having the motor rebuilt, which was a considerable amount. He finally called and spoke with me about the disappointing results, so I told him to have the car tuned on a chassis dyno. Two weeks later he called back to say thanks. He told me that he finally decided to follow my suggestion and had the car tuned on a chassis dyno. When they did the baseline pull, he was only making 105 RWHP, which was better then a stock motor, but not what he was hoping for. When they finished tuning the carb and ignition system, and hour later, he was pulling 145 RWHP. That's a 40HP gain, just from tuning the motor properly. Not bad for $150 bucks. Now the car runs great and he's very happy with the results. The car runs better than he expected, and he's getting 22 mpg. Moral of the story..... do your self a huge favor and have it tuned on a chassis dyno. I guaranty you won't be sorry.

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