Carburetor Tuning Secrets - Taming A Demon

The main jet is used to control the amount of air flowing through the carburetor's main circuit, while the air bleeds control the shape of the fuel curve. To get a better idea of how these components function, Schriefer says to imagine a funnel. The small end of the funnel represents fuel flow at idle, and the large end of the funnel represents fuel flow at WOT. Changing the main jet will change the size of the funnel, or the size of the holes at both ends, while changing the bleed will change the shape of the funnel, or the shape of the transition from the big end to the small end. Schriefer cautions, however, that you should only work with the air bleeds and emulsion circuitry when you have the tools, such as a real-time air/fuel meter (not the portable one mentioned earlier), to measure the results of the changes you are making. A small change here can cause serious engine damage if you aren't careful.

"We all know how to set the floats in a carburetor, but there are subtle changes you can make in the float level that can affect the way an engine drives," Schriefer says. "The higher the float level, the quicker the engine will start to draw fuel from the boosters. This can help tremendously with driveability." Quick trick: Raise the float levels until fuel drips from your boosters, then drop it back down until it stops.

One tuning change that helps driveability at part-throttle is working with the size of the power valve channel restrictors (PVCR). A larger PVCR allows more fuel to pass, which means you can run less main jet. This gives you a leaner part-throttle while maintaining proper WOT air/fuel ratio. When experimenting with this, you won't need to change much. Just increase the restrictor by one number or two to begin. Remember, anytime you make changes (not just carb changes), make the change and then test it. Don't make two or three changes and then test-you'll never be able to properly analyze the results.

Schriefer says you shouldn't only look at spacers as a way to boost peak power. They can also be used to move the engine's powerband up or down in the rpm range to make it more useable with your gearing at a particular racetrack. There are no hard-and-fast rules that work for every engine package when it comes to spacers, but there are a few general guidelines you can use to help determine your needs. The greatest role of the spacer is to increase the distance between the bottom of the carburetor and the roof of the plenum in the intake manifold. This allows a straighter path of flow for the air/fuel charge-which helps keep the fuel droplets suspended in the moving air-and also increases the plenum volume. Generally, adding plenum volume will increase top-end power at the expense of some bottom-end power. Reducing the plenum volume (by using a shorter spacer or switching from an open to a four-hole spacer) will help bottom-end torque while taking away some of the engine's ability to make power in the upper rpm range. Decreasing the plenum volume also increases the engine's signal to the carburetor and may, in some cases, help throttle response. Ask questions about the track on which you run. Is top-end power what you need, or is bottom-end torque, where you can really hustle off the corners, more important? The analysis of what you need on the track will direct you to the changes you need to make in the carb.

The spacer's design can also influence the relationship between the carburetor and engine. A four-hole spacer increases the distance between the carburetor and the plenum roof without adding more plenum volume than necessary. The holes are sized the same as or just slightly larger than the venturis and are helpful when it comes to straightening out the air/fuel flow at part-throttle, when the butterflies are at an angle to the air path. This makes a four-hole spacer a popular choice on tight or slippery tracks, when the driver spends a good portion of the lap at part throttle.

A tapered spacer begins as a four-hole spacer and transitions into an open spacer at the bottom. It isn't legal in many racing classes, but the tapered four-hole is by far the most popular choice for classes in which it is legal. The transition gives the spacer the ability to boost both low- and high-end power because the transition creates a venturi effect that actually serves to increase air speed through the carburetor-and as we've already discussed, greater air speed or velocity through the carburetor almost always equals better fuel atomization.

"Hopefully, I've given you some areas to look at that will help your program," Schriefer says. "Remember, there isn't going to be an end-all answer in each situation. The bottom line with carburetion is if you're having a problem, try to isolate when it's happening, determine which circuit(s) in the carburetor [is/are] being used, and add fuel. This will at least let you know if you're too rich or too lean, and where you can go from there. Please make sure to keep good notes when you're tuning so you can go back if the problem is not any better."