The carburetor system mixes the fuel and air before this mixture enters the intake manifold. The image below describes the operation of a float type carburetor system commonly seen in many general aviation aircraft.
Carburetors are normally calibrated at sea-level air pressure where the correct fuel-air mixture ratio is established with the mixture control set in the FULL RICH position. However, as altitude increases, the density of air entering the carburetor decreases, while the density of the fuel remains the same. The mixture control allows a pilot to manually adjust the amount of fuel mixed with intake air to compensate for varying atmospheric density.
As an aircraft increases in altitude, the surrounding air density lessens. As a result, if left unchanged, the resulting fuel-air mixture will become RICH (fuel heavy). The engine may begin to run rough, due to spark plug fouling from excessive carbon buildup on the plugs. Carbon buildup occurs because the rich mixture lowers the temperature inside the cylinder, inhibiting the complete combustion of the fuel. To correct for this the mixture must be leaned using the mixture knob. During a descent from high altitude, the fuel-air mixture must be enriched, or it may become too lean (oxygen heavy). An overly lean mixture causes detonation, which may result in rough engine operation, overheating, and/or a loss of power.
One disadvantage of the float-type carburetor is its icing tendency. Carburetor ice occurs due to the effect of fuel vaporization and the decrease in air pressure in the venturi, which causes a sharp temperature drop in the carburetor throat. This restricts the flow of the fuel-air mixture and reduces power. If enough ice builds up, the engine may cease to operate.
Carburetor heat is an anti-icing system that preheats the air before it reaches the carburetor and is intended to keep the fuel-air mixture above freezing to prevent the formation of carburetor ice. The carburetor heat should be checked during the engine runup however, limit use on the ground as the air typically bypasses the intake filter. Refer to your AFM/POH for details.
When conditions are conducive to carburetor icing during flight (as shown below), periodic checks should be made to detect its presence. If detected, full carburetor heat should be applied immediately, and it should be left in the ON position until the pilot is certain that all the ice has been removed. The use of carburetor heat causes a decrease in engine power, sometimes up to 15 percent, because the heated air is less dense than the outside air that had been entering the engine. The use of carburetor heat also enriches the mixture. When ice is present in an aircraft with a fixed-pitch propeller and carburetor heat is being used, there is a decrease in RPM, followed by a gradual increase in RPM as the ice melts. The engine also should run more smoothly after the ice has been removed. When carburetor heat is used on an aircraft with a constant-speed propeller and ice is present, a decrease in the manifold pressure is noticed, followed by a gradual increase.