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Chevy Carburetor Tuning Tips - Dial In Your Carb
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Problem: My Downleg Boosters Look So Antiquated. Should I Upgrade To Annulars?
Solution: Annular boosters provide superior low- and midrange drivability compared to straight and downleg boosters due to their improved atomization of fuel. Likewise, since they're larger and occupy more space in the venturi, annular boosters also amplify carb signal. While those characteristics are desirable in low-rpm street motors, they result in a rich condition that's difficult to control at the top of the powerband in high-rpm 'strip motors. "Annular boosters become restrictive at high rpm, so you're better off with a downleg booster in a street/'strip or race application," says Vrbancic. "On the other hand, annulars work great for most low-rpm street cars." Moreover, booster selection needn't be an either/or proposition. Using annular boosters on the primary side and downleg or straight boosters on the secondaries can yield excellent low-speed drivability in addition to precise control of the fuel curve at high rpm. Problem: My Car Hesitates When It Dead-Hooks Out Of The Hole.
Solution: If your car hooks up hard enough to uncover the jet at launch, setting the float level higher-about 3/4 up the sight glass-is a quick fix. An even better solution is installing a set of jet extensions. "Once you hit the 1.5-second, 60-foot range, uncovering the jets becomes very common," says James. "That said, we recommend extensions in all drag racing applications, because regardless of your 60-foot times, you never know how a car is going to pitch at launch." Problem: My Throttle Plates Are Almost Completely Closed, But My Motor Idles Too High.
Solution: In this scenario, air is entering the motor from a source other than the carburetor, which suggests a leak at the intake manifold. "Overtightening the manifold bolts, gasket misalignment, and using the wrong gaskets can all cause leaks," says Brown. "It's a very common problem that results in low manifold vacuum, a high idle speed, and lots of fumes and hydrocarbons out the tailpipe." Murphy estimates that 60 percent of all the cars he works on have a manifold leak. "A small leak may only kill one cylinder, which makes the motor idle rough, but everything smoothes out as the increase in rpm and air velocity moves enough fuel to cover it up," he says. Problem: How Do I Know What Power Valve To Use?
Solution: With motors that produce at least 12 inches of vacuum, a 6.5 power valve will work fine. If your motor pulls less than 12 inches of vacuum, the vacuum reading should be divided in half. For instance, if the vacuum gauge reads 9 inches, a 4.5 power valve is recommended. "A lot of time people change the power valve without knowing what it's doing," explains Vrbancic. "People often try to adjust the idle mixture screws when the actual source of excess fuel is a mismatched power valve that opens too soon. If the vacuum is jumping all over the place on the gauge due to a long-duration cam, keep the power valve [number] farther away from the vacuum signal. For example, with 10 inches of vacuum, that means going with a 4.5 power valve instead of a 5.5." Problem: Does It Matter That My Car Hasn't Got Much Hood Clearance?
Solution: Adequate hood clearance is essential to healthy carb operation. Holley recommends at least 3/4 inch of clearance between the carb and the hood or between the air cleaner and the top of the vent tubes. "Without adequate clearance, the fuel bowls will not be able to vent, which will cause pressure buildup in the fuel bowls," explains McFarland. "Instead of the engine drawing in fuel, this pressure will push fuel out of the boosters, resulting in an extremely rich condition or even engine stall."  The fundamental purpose of...  The fundamental purpose of a booster is to increase air flow through the venturi. An annular booster (left) provides superior atomization compared to a straight (right) or downleg booster at low rpm for improved throttle response. However, they tend to run rich and become restrictive at high rpm. According to QFT, for a given cfm rating, annular boosters can flow up to 10 percent less than their straight and downleg counterparts. Consequently, one style booster isn't necessarily better than another, and every application calls for a different style booster. They're easily removable on Holleys and Carters, while the boosters on a Q-jet are actually cast into the body of the carb.  To prevent a bog at WOT when...  To prevent a bog at WOT when manifold vacuum and air speed drop to near zero, the accelerator pump squirts fuel into the venturi until carb signal picks up once again. The throttle angle at which the fuel is squirted and the amount of fuel delivered can be tweaked by changing the pump cam and the size of the pump itself. They are positioned underneath the fuel bowls on a Holley, and on the driver-side front (beneath the air horn) on Carters and Q-jets.  Positioned next to the boosters...  Positioned next to the boosters in a Holley, the high-speed air bleeds allow further fine-tuning of the air/fuel mixture. The larger the bleed, the leaner the mixture, and the smaller the bleed, the richer the mixture. The only way to accurately gauge the effects of changing air bleeds is to precisely measure a motor's BSFC on a dyno. Likewise, since installing the wrong air bleeds can result in catastrophic engine failure, 99 percent of hot rodders are better off leaving them alone. In fact, Pro Systems says that customizing the diameter of the fuel passages is a far more precise and effective means of altering the fuel curve than changing the air bleeds.
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