Chevy Carburetor Tuning Tips - Dial In Your Carb

Problem: My Motor Feels Lazy After Each Shift, Then Picks Up Again.
Solution: To prevent sluggish shift recovery and the resulting slower e.t.'s, properly sizing the fuel channel passages in the metering blocks is essential. If the channel area is too large on a high-horsepower drag car, the jolt following each shift will cause a lean hesitation. "Billet metering blocks do not respond as fast to shift recovery as cast blocks," explains James. "That's because their fuel channels are cut with an end mill, and they end up being too large. The smaller you can keep the channel area, the faster it can respond after shifts, and reducing the cross-section of the channels by filling them with epoxy improves shift recovery dramatically. The problem is far less pronounced with cast blocks, since their fuel channels are smaller to begin with."

Problem: My Carb Loses Signal When I'm On The Bottle.
Solution: For most street applications where nitrous is limited to 150-200 hp, an out-of-the-box carb will work just fine. However, there's only so much volume beneath the plenum, and once nitrous dosages eclipse the 300-400hp mark, the nitrous displaces lots of air, in turn reducing carb signal. A simple fix is dropping down the size of the carb a tad, but every application is different. "Sometimes we'll reduce the size of the venturi, in other instances we'll increase the size of the booster, and some motors require doing both," explains Vrbancic. "It really depends on the specific engine combination, but the main goal is preventing a lean condition and keeping the signal strength up."

Problem: I Don't Know How To Adjust My Four-Corner Idle System.
Solution: A trained ear can adjust the idle air/fuel mixture screws by sound, but there's nothing wrong with relying on a vacuum gauge. "What you're trying to do is look for the highest point of vacuum as you turn the screws, inward to lean the mixture and outward to richen it up," explains Campbell. "Start at driver-side front and go around clockwise, then go around a second time to fine-set those screws. Since the front screws also feed into the idle circuit, they should be out even with or slightly more than the back screws. You want the screws to be as even as possible, within 1/2 to 1/4 turn of each other."

Problem: I Have To Starve For A Week In Order To Afford The Gas For Cruise Night.
Solution: Although carb tuning can't make up for fundamentally poor engine and driveline design, it can yield gains of a couple of mpg. "Run as small of an accelerator pump as possible so you're not squirting any more raw, unatomized fuel into the motor than you have to," advises Murphy. "Outside of drag motors, most cars don't need that much fuel off-idle. If you have a wideband O2 sensor, you can tune your carb to run as high as a 16:1 air/fuel ratio at cruise without any detrimental drivability effects. The more vacuum advance and initial timing you run, the more vacuum the motor will produce, so the less throttle you'll need. With these simple tricks, we've had customers pick up 2 to 3 mpg." Moreover, since Quadrajets and Edelbrock/Carter carbs utilize a tapered metering rod-that can precisely meter fuel at part throttle-in addition to jets, they typically offer better gas mileage than modular Holley designs.