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 carb 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 to customers who have purchased a Weber carb from Classic Inlines. This kit will include several jet sizes, emulsion tubes, air bleeds and/or corrector jets, and an A/F Ratio Gauge. Obviously a sizable 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). The A/F Ratio Gauge is an invaluable tool for tuning your carburetor, but it's generally too expensive ($500) for most customers to purchase, especially when you consider it's only use once or twice. Therefore we have decided to make them available to our customers for a very modest fee. Click here for more info.

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 not see a significant loss in fuel economy, over stock, but you should see 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. As such, you need to make sure your fuel system supplies an adequate amount of fuel, at the proper pressure, and that your ignition system is working properly. You also need to make sure that the ignition system is capable of supplying an adequate spark to burn the extra fuel, and that the distributor was designed to work with a Weber carb.

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.

Fuel Delivery
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 3-4 psi. The Carter 4070, which is a rotary pump, is an excellent choice. This pump does not need, and should not use, a pressure regulator for any reason.

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 of fuel, keeping the bowl full at all times.

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. You guarantee you won't regret it.

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 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 anywhere 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.

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, alter the characteristics of an emulsion tube, and affect driveability and fuel consumption.

Float level, in mm, typically refers to the distance from the face of the carburetor top cover to the float. With the top cover held vertically (float pivot at the top) and 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, to 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.

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 best 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. You can also tune your car on a chassis dyno, but this can get pretty expensive too. 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 threads of the plug. Remember, a new set of plugs may take a few minutes to get some color on it, 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 AS THIS WILL DAMAGE THE SCREW AND IT’S SEAT 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 screws 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 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. Remember, your 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 you have the idle circuit right then the primary main jet is too small. Drive the car above 2000-3000 RPM for a few minutes then shut the engine off before letting it idle, then check the plugs. Do not let it idle when checking the spark plug color for the main jets.  If the car transitions fine but hesitates or falls on it face when the secondary is opened you need to work on the secondary main jet. 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.

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 large air correctors.

Diagrams (click on image)
32/36 DGV	38/38 DGS	DCOE

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
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 styles of idle jet. 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

BE SURE TO FOLLOW THE INSTRUCTIONS IN THE PROPER SEQUENCE. DEVIATION MAY CAUSE THE CARBURETOR NOT TO FUNCTION TO ITS IDEAL SPECIFICATIONS AND IT MAY NOT PROVIDE THE PERFORMANCE AND/OR FUEL ECONOMY YOU DESIRE