Chevy Carburetor Tuning Tips - Dial In Your Carb

Problem: My Idle Circuit Must Be Messed Up, Because The Idle Is Way Too Rich.
Solution: When a motor runs rich at idle even with the right size power valve and the adjustment screws turned all the way in, it may be time to bust out the fuel pressure gauge. "The biggest problem we encounter in our shop is cars with excessive fuel pressure," explains Murphy. "Carbs love volume, but hate excessive pressure. The most pressure you want to run is 7 psi, and 10 psi is excessive. Design a fuel system with minimum pressure and maximum volume." Things get a bit trickier with a big cam, as low vacuum requires opening up the throttle blades to maintain a steady idle. This exposes more of the transfer slot, resulting in a rich condition. McFarland advises that no more than 0.025 to 0.030 inch of the transfer slot should be exposed past the throttle plate at idle. To compensate, the secondaries can be cracked open slightly or two small holes can be drilled on the primary throttle blades. Start at 1/16 inch, then drill progressively larger holes until the desired idle speed is reached. Barry Grant even offers carbs with holes already in the baseplate, eliminating the need to drill.

Problem: My Motor Hesitates, Backfires, And Won't Hold A Steady Idle.
Solution: Many hot rodders crank up the idle circuit to richen up the air/fuel mixture in this situation, but that's the last thing you want to do. Chances are these symptoms are caused by too much cam and not enough timing. "Big cams and compression don't produce much cylinder pressure at idle, so there's no way a motor will run properly with only 8 degrees of initial timing," says Vrbancic. "You can't expect a performance motor to run well with the same timing as with a stock camshaft. Simply turning the timing up to 15 degrees to compensate for the low vacuum usually fixes the problem entirely."

Problem: I Spec'd My Carb Using A Formula Out Of A Book, But I Think It's The Wrong Size.
Solution: Proper sizing is half the battle when dialing in a carburetor, and our expert panel unanimously agrees that the formulas found online and in books border on useless. "If you follow those formulas, everyone in Pro Stock is running carbs that are too small," quips James. "A big-inch 600hp motor that doesn't turn a lot of rpm requires a vastly different carb than a small-inch 600hp screamer even though they both make the same power. Make your decision based on what size carb others have used successfully in applications similar to yours, and keep extensive records of what works and what doesn't." Likewise, according to Campbell, sizing a carb based on cam duration at 0.050 can greatly simplify the process. "There are so many variables when sizing a carb, such as the power level, displacement, rpm range, transmission type, converter stall speed, vehicle weight, and so on," he says. "Since cam selection also accounts for all these variables, sizing a carb based on cam duration is a good approach."