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. It can run too hot, overheat, and cause a lack of power or 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. There are several different methods to determine if the air/fuel mixture is correct. Among them are the following:

1. Read the spark plugs using an illuminated magnifying glass. This method involves looking at the base of the spark plug insulator (white part of the plug) for a slight coloring on the insulator just above where the insulator comes through the steel case. If the mixture is too lean, it will leave no color. A rich mixture causes the fuel ring to become more prominent. Over-rich mixtures give the plug a sooty appearance.

Another way to determine air/fuel mixture is by pulling the header off and looking at the color of the exhaust port in the cylinder head and at the first 6 inches of the exhaust header. However, the header and spark plug color can only show what the air/fuel mixture was at the load condition where you did the check. In the days of leaded fuel and point ignition, this method worked quite well. Today's use of unleaded fuels and high-energy ignition systems has made this method much harder because very little color is seen on the spark plug, making this a job for an expert.

2. Use timed acceleration runs or top speed for the power system. This involves using trial-and-error jetting changes to obtain the best results. Obtaining the correct cruise mixture (which is the air/fuel mixture while the engine drives under light load conditions such as pace laps and yellow flag conditions) is not as easy since it involves jetting the carburetor to get the highest vacuum, then trial and error to get the best engine drivability. When setting the power and cruise mixtures, it's always advisable to stay a little rich in order to avoid engine damage. The idle mixture is set using a tachometer to get the max speed from each idle screw and then go leaner to get a 20-rpm drop. This is known as the lean drop method.

3. The easiest and most accurate method we've found is with the use of an infrared exhaust gas analyzer like the unit we use from OTC/SPX Tool Company called the Perfor- mance Gas Module. This unit allows us to determine what the air/fuel mixture is by "reading" the exhaust gases. By using the Performance Gas unit, the carburetor's jetting (air/fuel mixture) curve can be checked at idle, cruise, or power loads, and then be tailored to what your engine needs to run at its best at all race/driving conditions.

4. An optional method of checking air/fuel mixtures is by using an extended range oxygen sensor installed in the exhaust header. The oxygen sensor is read using a unit referred to as a Lambda meter. This method looks at the unburned combustibles in the engine's exhaust. The unit supplies an air/fuel mixture reading. The readings are very accurate, but false readings can be created by an exhaust leak, engine misfire, or a high overlap camshaft at lower engine rpm. All will cause extra oxygen to be in the exhaust, creating an inaccurate air/fuel mixture reading.

The most accurate and easiest way to check the jetting (air/fuel mixture) of an engine is by observing the carbon monoxide (CO) reading from any infrared gas analyzer, such as the portable unit we are using. To find this value, place the sample probe into the tailpipe, and then the unit will "read" the exhaust and supply the readings necessary to determine the air/fuel mixture. The infrared exhaust gas analyzer and the Lambda meter method allow part throttle fuel mixtures to be checked, which would otherwise be nearly impossible. The readings from either method can be recorded and later played back or looked at in real time at the track or on a dynamometer. It is important to note that any changes other than jet changes and basic adjustments should be performed by a proficient carburetor expert.

After the basic engine condition and tune-up (fuel pressure, timing curve, and so on) are confirmed to be correct and there are no vacuum leaks, the next step is to determine the air/fuel mixture at idle through 3,000 rpm. If the cruise mixture is off, change the jets to get the air/fuel mixture correct at 2,500-3,000 rpm, or cruise range. Next 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 1,000-1,800 rpm, the idle channel restriction, if used, may have to be slightly enlarged to allow more fuel to be delivered at part throttle.