Savvy engine builders have for years been using clues such as burn patterns on the spark plugs and even the color at the exhaust pipe exits to tell them how the carburetor should be tuned. But if you don't have 20 years of engine-building experience, you can rely on testing and diagnostic tools.
To get good results, you don't have to invest hundreds of thousands of dollars in wet-flow benches and other exotic pieces of equipment, such as those Schriefer has available at Barry Grant's headquarters in Georgia and its R&D facility in North Carolina. One invaluable tool you should consider is a portable air/fuel meter, several of which have recently shown up on the market at very affordable prices. Both F.A.S.T. and Innovate offer small, portable units that are perfect for racers, as they are easily installed for testing and removed again for race day. A good air/fuel meter hooks up with sensors in the exhaust headers and will tell you your air/fuel ratio at every point along your engine's rpm range. Most record readings and play them back later. Some hook up to a laptop and allow you to store your findings. For more information on tuning with a F.A.S.T. air/fuel meter, check out "Better Measure" (Apr. '07 Circle Track).
Schriefer says the best place to start when tuning a carburetor is with the idle settings. "The first key to getting an engine to idle and drive properly is butterfly positioning. If the butterflies are open too far, they will expose too much of the transfer slot," he says. "This will cause the engine to draw fuel from the slot, giving poor throttle response, acceleration, fuel economy, and performance.
"I always find it amazing to watch a race and see flames shoot out of the exhaust when the driver is off the throttle. Ninety-nine percent of the time, with a carbureted car, it is because the transfer slots are open too far."
Every motor is different, so there is no single setting for the butterflies and transfer slots to achieve optimum idle. Generally, you only want approximately 0.020 inch of the transfer slots exposed. This means that the area of the slots you can see basically forms a square. That being said, different cam profiles mean engines will require different amounts of air to idle properly. Proper idle speed, while usually between 800 and 1,200 rpm, is really the point at which your engine will idle reliably and crank consistently. But getting your engine to optimum idle isn't just a matter of adjusting the butterflies to allow enough air through the carburetor when the driver has his foot off of the throttle. For example, if you are using a distributor with a mechanical advance mechanism, you will need to adjust the timing at idle to achieve a good, steady idle. When doing this, you don't want your timing at full advance to change. Consequently, you will have to work with your advance mechanism to get both the idle timing and full-advance timing constant.
The most common method for carb tuners to tune idle speed is to drill small holes in the butterflies to allow air to pass while still keeping the correct amount of the transfer slots exposed. The problem with this is once the hole is drilled, it is there forever. Putting this carburetor on a new engine can prove to be a problem. The only tuning options you have are to drill the hole bigger or pony up for a new set of butterflies if you wish to go smaller.
Schriefer prefers instead to install a jet in the center of the baseplate. It achieves the same goal as drilling, but changing the amount of airflow when the butterflies are closed is now as simple as unscrewing the existing jet and installing one with either a larger or smaller hole. Not only is this simpler, it is also a very easy change to make at the racetrack, unlike drilling the butterflies. Barry Grant's Race Demon carburetors are manufactured with this as a standard feature. However, it is possible to add a jet in the center of the baseplate for most other four-barrel carburetors. But it is slightly more involved than simply drilling and tapping a hole in the center of the baseplate. You must also cut grooves in the baseplate from each of the venturis to the hole because this area of the baseplate is normally sealed off from the airflow in the venturis. Although it certainly can be done, this is probably the type of project best reserved for a carb specialist.
Once the butterflies are properly set, you can move on to the idle mixture. Schriefer has four tips for working with the idle mixture screws:
Different engine packages require different amounts of air to idle properly. On race engin
1. Take your time; the engine must draw the fuel through the carburetor.
2. Adjust the mixture at the temperature you're going to run the engine. In other words, make sure the engine is up to typical race-operating temp before you start changing anything. Adjusting the mixture at 160 degrees when your typical race temp is 190 will result in tuning right into a rich condition. That's a waste of fuel and a loss of power.
3. Don't be afraid to go through the mixture screws multiple times. Record each adjustment and note the results. This is the old "measure twice, cut once" principle.
4. Remember your goal: get the engine to idle as high as possible with the butterflies as closed as possible.
If you own an air/fuel meter, setting the idle screws is simple: just adjust until you get between a 13.5:1 and a 14:1 reading, which is slightly rich but still safe. If you don't have the luxury of a meter, you should adjust the screws until you have a steady idle and the engine doesn't stumble when the driver picks up the throttle.
And finally, here's one tip to help keep your engine safe and healthy. Schriefer says that if you question any adjustment you've made, always ensure the air/fuel mixture is too rich rather than too lean. A lean condition can quickly burn pistons, but a rich condition will only hurt power. Obviously, this tip is applicable just as you begin the tuning phase. If you start rich and work to optimum, you won't risk damaging your motor.