Chevy Carburetor Tuning Tips - Dial In Your Carb

Problem: I Just Installed A Centrifugal Blower, And Now My Carb Can't Keep Up.
Solution: Maintaining a consistent fuel curve with a blow-through supercharger arrangement is particularly difficult due to the rapidly changing fuel demands of a motor that cycles in and out of boost. The key is to resist the urge to plop a big carb on the intake to ensure good drivability off boost, and to modify the carb's internals for additional fuel flow under boost at high rpm. According the Brown, a 750-cfm carb can easily support 1,000 hp in a blow-through application. In addition to installing annular boosters that increase fuel flow at low rpm, just about all the internal fuel passages must be increased in size to support additional fuel volume. "In our blow-through carbs, we make the main wells, fuel passages, and needle-and-seat assemblies larger," he explains. "The fuel system needs to be addressed as well, since high volume is a good thing but high pressure can aerate the lines." Additionally, machining lip seals into the baseplate prevents pressurized air from escaping in high-boost applications. Van Noy has been at the forefront of blow-through carbs for years, and has a few extra tricks up his sleeve. "We make our own boost-referenced power valves that only open up until you hit a predetermined boost level and a custom booster that enhances signal at low rpm," he says.

Problem: I Want To Impress My Buddies With A Custom Carb, But I'm Not Sure If I Need One.
Solution: Factory carb calibrations work great for the vast majority of applications, but the bigger the cam, the more you can benefit from a custom-tuned carb. "All universal carbs are just that, universal," explains McFarland. "Although we strive to get the best calibration as possible for most engines, and they work very well out-of-the-box for most applications, there is always a little more to be had by tuning a carb specifically to your engine's needs." Once duration numbers are in the mid-240- to 250-degree range, engines become more finicky and often require constant tuning. "You can expect to pick up one- to three-tenths in the quarter with a custom carb in a typical street/'strip car, but an even bigger benefit is the improved idle quality, throttle response, reliability, and consistency," explains Murphy. "Once a custom carb is set up, all you have to adjust are the jets."

Problem: Whenever I Pop My Hood, People Laugh At My Vacuum-Secondary Carb.
Solution: Double-pumpers look tough and just about every race car ever built has one, but they aren't always the best choice for most street applications. If torque converter stall speed and lockup aren't properly matched to cam duration, mechanical secondaries will often open too soon, killing precious vacuum and air speed when the motor needs it the most. "For most street cars and even many street/'strip applications, a vacuum-secondary carburetor works best," says McFarland. "They work very well on midweight or heavyweight cars with an automatic transmission. Since they operate by sensing engine load and only open when the extra fuel is needed, they are more forgiving than mechanical secondary carbs. The mechanical secondary carburetor is best suited for lighter cars with radical camshafts, short gears, and manual transmissions, or for full-blown race cars."