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Assuming you read the introduction in Chapter One and followed the idle circuit setup guide in Chapter Two, it’s time to move on to the primary and secondary circuits. Again we want to do this in the correct order, as each circuit progressively affects the fuelling, and thus the way the engine runs. In other words, you want to set the idle circuit up before playing with the primary circuit. By the same token, you need to dial the primary circuit in before you mess with the secondary circuit.
The DGV series is a progressive two-barrel carburetor, meaning that the primary 32mm butterfly opens first when you move the linkage (or press the accelerator), followed by the secondary 36mm butterfly as you continue to open the throttle. You can feel the increased resistance when the linkage starts to open the secondary butterfly. This resistance makes it clear whether you are operating on the primary barrel only or have started to open the secondary.
The primary and secondary circuits are each metered by a main jet and an air corrector, sometimes called an air jet. The main jet is used to set basic fuel flow volume. The air corrector is used to adjust that fuel volume as airflow through the venturi increases with increased engine speed and load. A larger main jet passes more fuel and produces a richer mixture. In contrast, air correctors (air jets) meter air, rather than fuel; a larger air corrector produces a leaner mixture.
You will have to change these parts in order to achieve the correct air-fuel ratio for your particular engine’s needs and your operating conditions. The Weber DGV also uses some other calibrated part, such as emulsion tubes, but most street engines do not require that those be changed. There is also an idle jet on the secondary barrel, but it doesn’t really have much influence and can typically be left alone.
As a reminder, the main jets are inside the float bowl, down at the bottom, toward the rear. The air correctors are at the top of carburetor body, just behind the float bowl. Both are located under the the float bowl lid, which covers the top of the entire body. You will have the float bowl lid off and on several times in the course of changing main jets and air correctors.
That’s enough theory. Let’s move on to tuning. The first thing we are going to look at is the primary main jet. This jet typically affects the fuelling from right off idle to halfway through the RPM range that you encounter without getting into the secondary barrel. Be careful to confine your tuning to low engine speeds only for now. Higher engine speeds begin to bring the influence of the primary air corrector into play.
A smaller number on the jet indicates a smaller hole drilled through it, and that means less fuel passes through the jet. In other words, smaller means leaner where main jets are concerned. Basically, you want to select the smallest (leanest) primary main jet that will run smoothly at low engine speeds on the primary barrel only. Continue experimenting with leaner primary main jets until the engine bucks or surges at low speeds and low throttle settings, such as cruising slowly around the neighborhood or maneuvering around a parking lot. Once you encounter bucking or surging, go up one or two sizes on the primary main jet. Our goal is to keep the air-fuel ratio just rich enough to be smooth and driveable. Anything richer than that is simply wasted fuel. We can stay relatively lean here and not put the engine in any danger since it is not under any significant load.
With the primary main jet chosen, you can move on to the primary air corrector, which admits air to emulsify the fuel (mixes it into a froth, more or less) as air velocity through the carb increases. The air corrector’s influence on the mixture increases as engine speed and airflow increase. In other words, the main jet sets the base fuel delivery on that barrel, and the air corrector trims the fuel delivery at higher engine speeds.
What you want to do is select the largest (leanest) primary air corrector that will run smoothly through the higher engine speeds when on the primary barrel only. On the primary, the engine will still not be under massive load, so you can stay fairly lean and still be safe. Verify that the car will cruise smoothly at high speeds and doesn’t surge, ping, or detonate when climbing hills or using normal acceleration on the primary throttle. If you’re using a wideband you can look for air-fuel ratios around stoichiometric, somewhere between 14.5:1 and 15.5:1 at steady cruise, and 13.5:1 or 14.0:1 under light acceleration and moderate load.
At this point, the vehicle should be smooth and responsive on the primary barrel under any conditions. Once the primary side is sorted you can address the secondary, but the primary side MUST be sorted first.
Moving on, the secondary circuit is calibrated in much the same way as the primary, but it’s important to remember how the secondary side of the carburetor is used. Most normal driving is done on the primary barrel, with the secondary being opened only for heavy loads or maximum acceleration. For that reason you’re going to want progressively richer air-fuel ratios when the secondary is open then you would when driving on the primary barrel only.
Secondary tuning also starts with the main jet. This time choose the smallest* size secondary main jet that provides smooth performance as the secondary starts to open, and midway through the RPM band, with the secondary barrel open. This is a good time to start paying attention to the wideband, if you have it, to monitor the air-fuel ratio. If the secondary is open, you generally want to see an air-fuel ratio between 12.5:1 and 13:1, going richer as speeds and loads increase. If you do not have access to a wideband oxygen sensor and air-fuel ratio gauge, proceed very carefully through leaner secondary main jets. At the first sign of surging, hesitation, detonation or pinging, go back up at least two sizes richer on the secondary main jet.
Finally, take a look at the secondary air corrector and choose the largest* size that provides smooth performance through the upper portion of the RPM range with the secondary throttle open. Be aware that we are now deep into a potential minefield. It is entirely possible to destroy your engine if you run it too lean under heavy load. Buying or borrowing a wideband oxygen sensor is practically mandatory if you want to establish the correct air-fuel mixture. As you did with the secondary main jet, go back at least two sizes richer (smaller number) on the secondary air corrector if you experience any surging, hesitation, detonation or pinging in the RPM and load band we are testing in.
*Please understand that tuning for WOT (wide open throttle) air-fuel ratios without a wideband oxygen sensor and air-fuel ratio gauge is very risky. In the days of leaded fuel, you could read the spark plugs to get an idea of how rich or lean the engine was, but that’s much tougher with unleaded gas. If you cannot afford to buy a wideband system, consider borrowing or renting one from a friend.
And there you have it – DGV tuning step by step. Not exactly cutting-edge technology, but sometimes it’s worthwhile to revisit things, and these are still popular and viable carburetors on a whole lot of older cars. Have fun and be safe!