Racing Carburetors - Jetting A Race Engine

Jetting a carburetor is one of the few "black arts" in the automotive world that is still a mystery to most racers and tuners. Most tuners still look at the spark plug, the exhaust port, and the first 6 inches of the header for proper color and make a guess at what jet size change is needed. One of the disadvantages of this method is that the header and spark plug can only indicate what the mixture was at the rpm and load condition at the time. A more scientific or modern method of checking the air/fuel mixture is the use of an infrared exhaust gas analyzer and/or an extended range oxygen sensor in the exhaust system. With this method, the fuel mixture can be "seen" at any rpm and load condition you wish to check. The content of the engine's exhaust can show the air/fuel mixture and how efficiently the engine is burning the fuel.

The proper tuning of a race engine can make the difference between being the winner and dealing with never-ending hassles, trying to keep up with the competition. For most racers, one of the biggest mysteries is jetting the engine in order to obtain the correct air/fuel ratio necessary for your race engine to supply drivable horsepower under all race load demands and while cruising during caution laps. Having the correct air/fuel mixture for the engine's needs while cruising at caution speeds is often ignored. If the air/fuel mixture is too rich for the engine while running at caution speeds, the engine may load up and foul the spark plugs. If the air/fuel mixture is too lean, the engine may run hot. Having an air/fuel mixture rich enough for all racing conditions will allow you to get all the horsepower out of the engine while getting as many laps as possible from a tank of fuel without overheating or doing engine damage that would normally result from having an air/fuel mixture too lean.

Achieving this is one of the many tricks it takes to beat the competition to the finish line. This may sound impossible, but new advances in exhaust gas analysis technology have made it possible to "read" and/or record the air/fuel mixture under almost any driving condition. In the past, exhaust gas analyzers have tended to be large and expensive. We have been using one from the PerformanceGas series of infrared exhaust gas analyzers from OTC/SPX. These units are not only compact and portable, but also affordable for a grassroots racer.

Most race carburetors sold today have a generic tune-up or jetting unless the carburetor is built for a specific engine package and fuel. Just adding mufflers or performing any header/exhaust system change (such as adding an "H" pipe into the exhaust) can cause the air/fuel mixture to change, making it necessary to rejet the carburetor. A carburetor not built and tuned for a specific engine, exhaust system, and fuel must supply an air/fuel mixture rich enough for a variety of engines. If the carburetor is supplying an air/fuel mixture that is too lean, the engine will run sluggish, overheat, or the lean mixture could cause engine damage. If the carburetor is supplying an air/fuel mixture that is too rich, the engine may tend to load up, foul the spark plugs, run sluggish, and lack power.

The fuel you use (race or pump), the air density (i.e., barometric pressure, air temperature, humidity), compression ratio, camshaft, exhaust system, ignition timing curve, engine condition, fuel pressure, and so on will all affect the carburetor tune-up needed to get the correct fuel mixture for your engine. If the rules require mufflers at one track but not at another, the carburetor tune-up will need to be adjusted for the change in exhaust backpressure.

The first order of business is to get the correct ignition advance curve for the engine and fuel being used. The fuel pressure must be checked to be sure it has the proper system pressure at all engine load conditions. If the fuel pressure drops below the proper pressure, the carburetor's air/fuel mixture will go lean and engine damage may follow. Once the ignition advance curve has been confirmed, many of the problems we see can be traced to a fuel mixture that is incorrect for the engine's needs.

Before checking the air/fuel mixture, ignition timing and advance curve must first be correct. Any distributor, performance replacement, or original equipment, must have the mechanical and vacuum advance curves checked and then tailored to the engine and the fuel being used. (Note: MSD distributors come with a very conservative mechanical advance curve. Bushings and springs are included in the box to get the desired curve.) An incorrect advance curve may cause an engine to lack power, ping, use too much fuel, or cause the engine to overheat. There are two methods we use to check the distributor's advance curve. One way is to put the distributor on a test stand. The second way is by using an advance timing light such as the Black Light series from OTC/SPX Tool Company.