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 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. The content of the engine’s exhaust can show what the air/fuel mixture is and how efficiently the engine is burning the fuel. Races are often won or lost by getting more laps out of a tank of fuel than the competition, how efficiently the fuel is burned by the engine can be a major factor in the difference between winning and losing a race.

The proper tuning of a race engine can make the difference between being the winner or having never ending troubles while trying to just keep up with the competition. For most racers, one of the biggest mysteries is how do you jet the engine in order to obtain the correct air to fuel ratio necessary for your race engine to not only supply drivable horsepower under all race load demands but also while cruising during caution laps, having the air/fuel mixture correct for the engines needs while you are cruising around the track at caution speeds is often ignored. If the air/fuel mixture is too rich for the engine while you are running at caution speeds, the engine may tend to load up and foul the spark plugs, while if the air/fuel mixture is too lean the engine may tend to run hot. Having a 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 any engine damage from having too lean of an air/fuel mixture, this is one of the many tricks it takes to beat the competition to the finish line. This may sound impossible, but the new advances in exhaust gas analysis technology have made it possible to read and/or record what the air/fuel mixture actually is under almost any driving condition. In the past exhaust gas analyzers have tended to be large and expensive, but we have been using one of new modern units 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 grass roots 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 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 re-jet 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 too lean of an air/fuel mixture, 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.

Ignition Advance Curve

Before checking the air/fuel mixture, the 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 and included in the box are the bushings and springs to get the desired curve.)

Air / Fuel Mixture

A lean fuel mixture (too little fuel for the amount of air in the cylinder) can cause an engine to stumble or give a rough idle as well as to run too hot, overheat, and cause a lack of power as well as engine failure. A rich fuel mixture (too much fuel for the amount of air in the cylinder) can cause an engine to load up at idle, foul the spark plugs, and also lack power or run sluggish…An optional method of checking air fuel mixtures is by using a wide band oxygen sensor installed into the exhaust header, the wide band oxygen sensor is read using a digital air/fuel meter. The unit that I have had the best results from is available from Innovate Motorsports. This method looks at the oxygen/unburned combustibles in the engine’s exhaust and then the unit supplies an air/fuel mixture reading; the readings are very accurate…

On Car Testing

After the basic engine condition and tune-up (fuel pressure, timing curve, etc) is confirmed to be correct, as well as checking to be sure there are no vacuum leaks, the next step is to determine what the air/fuel mixture is at idle through 3000rpm. If the cruise mixture is off, first change the jets in order to get the air/fuel mixture correct at the 2500-3000-cruise rpm range. Then check and set the idle mixture. If the air/fuel mixture is too lean at idle or part throttle and the idle mixture screws do not provide enough adjustment, the correction may involve enlarging the idle jet. If the mixture is still lean at 1000 through 1800 rpm, the idle channel restriction, if used, may have to be slightly enlarged to allow more fuel to be delivered at part throttle.

This lean condition at part throttle condition will cause the engine to miss or stumble is due to the lean air/fuel mixture. This problem is very common on many carbs. (The notable exception is the Demon line of carburetors, which have used the PerformanceGas to do exhaust gas analysis along with dyno, track and drivability testing to establish the factory fuel curve that allows them to come with a stumble-free guarantee.) If the air/fuel mixture is too rich at idle and part throttle, the idle jet/restriction may be too big and may need to be replaced with a smaller one.

A carburetor has an accelerator pump, idle, main jets, and in most cases a power system that is designed to supply the correct air/fuel mixture for the engine’s demands. An idle system will have an idle jet/restriction that must be changed to supply the desired fuel mixture for idle and off idle engine demands. For a carburetor that uses a power valve, the main jet size is what determines what air/fuel mixture is delivered to the engine at light load/cruise speeds (1500rpm and up). The power valve restriction (under the power valve) is the determining factor in what the air/fuel mixture the carburetor will supply when the power valve is open; a 6.5 inch power valve will be open and supplying the richer air/fuel mixture needed under high power demands anytime the vacuum is below its 6.5 opening point. A carburetor that uses metering rods in the primary jets such as a Quadrajet, will use the metering rods to change the air/fuel ratio for both the power and cruise mixture demands of the engine; the larger the metering rod diameter the leaner the air/fuel mixture will be. The accelerator pump adds fuel as the throttle valves are opened, tuning the accelerator pump squirter volume and duration it’s tuning is mainly trial and error.

The Lambda meter method uses an extended range oxygen sensor to determine the fuel mixture by analyzing the unburned combustibles in the exhaust gas, a extended range oxygen sensor can read air/fuel mixtures as rich as 10 to 1 or on the lean side it can read air/fuel mixtures lf 19 to 1 or leaner (a standard oxygen sensor is only accurate at air/fuel mixtures of around 14.7 to 1)…The Innovate Motorsports Digital Air/Fuel Meter can allow you to sample and record the air/fuel mixture data at a rate of 12 samples per second over a period of up to 44 minutes, this data can allow you to tune the fuel mixture to the ideal air/fuel mixture curve that the infrared exhaust gas analyzer can help you establish.

Tuning at the track

When the engine is being used at a racetrack or anytime you are trying to get the very most from the engine, the air/fuel mixtures should be adjusted to correct for the ever changing air density. First, the baseline tune-up for the air/fuel mixtures must be set and correct for the conditions you have tested under, then the next step is fine-tuning for changes in the air density….

For the complete text of this article, please visit Circle Track Magazine