The mixture control regulates the fuel-to-air ratio. The fuel/air ratio is the ratio between the weight of the fuel and the weight of the air entering the cylinder. As altitude increases, the amount of air in a fixed volume (air density) decreases, so the ratio is between weights, not volume. About 15 lbs. of air is needed to unite with 1 lb. of gasoline to completely combine all of the gasoline and oxygen. If there is more air than is needed (mixture too lean), oxygen will be left over after the burning is completed. If there is more fuel than is required for the amount of oxygen (mixture too rich), free carbon will be left—this usually shows up as black smoke or soot. A mixture that is too rich may lead to rough running, spark plug fouling, increased fuel consumption, and a loss of power (loss of RPM for a fixed-pitch propeller and a loss of manifold pressure for a constant-speed propeller). A mixture that is too lean will cause excessively high cylinder head temperatures, leading to abnormal combustion (detonation) or engine stopping. However, Too rich is preferable to too lean.

At high power settings (in excess of 75%), a full-rich mixture is necessary to provide excess fuel as a coolant. Pilots are taught to use Full Rich mixture for takeoff, but this is not the case at high density altitudes. At density altitudes above 3000 feet, the mixture should be leaned for takeoff. On a very hot day, even at only 1000 ft MSL, the atmosphere may have a density altitude of several thousand feet that requires leaning the mixture.

To test for an overly rich mixture before takeoff, set the RPM to 1100 and slowly pull back the mixture control while observing the tachometer. You should get no more than a 50 RPM rise before the engine begins to quit. A higher RPM rise means the mixture is too rich.

To establish the proper mixture ratio while airborne, set the power for cruise flight by adjusting the propeller pitch control and the throttle. Lean the mixture until the EGT of the hottest cylinder peaks. Then, move the mixture control back toward rich until the EGT drops about 15 degrees F. A leaner mixture results in an increase in EGT because leaner mixtures burn slower, which causes the exhaust gases to stay hotter for an extended period of time. Not leaning enough at altitude can cause fouled plugs, which will prevent the fuel/air charge from burning as quickly or efficiently. The mixture should be moved to the RICH position at the bottom of the descent, or progressively richened if descending from a high altitude.