A “mixture control” in the cockpit is provided to change the fuel flow to the engine to compensate for varying air densities as the airplane changes altitude.

 Carburetors are normally calibrated at sea level pressure to meter the correct amount of fuel with the mixture control in a “full rich” position. As altitude increases, air density decreases. This means that a given volume of air does not weigh as much at higher altitudes because it does not contain as many air molecules. As altitude increases, the weight of air decreases, even though the volume of air entering the carburetor remains the same. To compensate for this difference, the mixture control is used to adjust the ratio of fuel-to-air mixture entering the combustion chamber. This also regulates fuel consumption.
 If the fuel/air mixture is too rich, i.e., too much fuel in terms of the weight of air, excessive fuel consumption, rough engine operation, and appreciable loss of power will occur. Because of excessive fuel, a cooling effect takes place which causes below normal temperatures in the combustion chambers. This cooling results in spark plug fouling. Conversely, operation with an excessively lean mixture, i.e., too little fuel in terms of the weight of air, will result in rough engine operation, detonation, overheating, and a loss of power.

 To summarize, as the airplane climbs and the atmospheric pressure decreases, there is a corresponding decrease in the weight of air passing through the induction system. The volume of air, however, remains constant, and since it is the volume of airflow which determines the pressure drop at the throat of the venturi, the carburetor tends to meter the same amount of fuel to this thin air as to the dense air at sea level. Therefore, the mixture becomes richer as the airplane gains altitude. The mixture control prevents this by decreasing the rate of fuel discharge to compensate for the decrease in air density. However, the mixture must be enriched when descending from altitude.

Follow the manufacturer’s recommendation for the particular airplane being flown to determine the proper leaning/enriching procedures.

Carburetor Icing  The vaporization of fuel, combined with the expansion of air as it flows through the carburetor, causes a sudden cooling of the mixture. The temperature of the air passing through the carburetor may drop significantly within a fraction of a second. Water vapor in the air is “squeezed out” by this cooling and, if the temperature in the carburetor reaches 0° C (32° F) or below, the moisture will be deposited as frost or ice inside the carburetor passages. Even a slight accumulation of this deposit will reduce power and may lead to complete engine failure, particularly when the throttle is partly or fully closed. [Figure 2-8] Conditions Conducive to Carburetor Icing  On dry days, or when the temperature is well below freezing, the moisture in the air is not generally enough to cause trouble. But if the temperature is between -7° C (20° F) and 21° C (70° F), with visible moisture or high humidity, the pilot should be constantly on the alert for carburetor ice. During low or closed throttle settings, an engine is particularly susceptible to carburetor icing.

Figure 2-8.—Formation of ice (white) in the fuel intake system may reduce or block fuel flow (red) to the engine.