The formation of ice in the carburetor has always been a problem in those airplanes equipped with a float type carburetor. It may restrict the power output of the engine or even cause the engine to quit operating. It is not a problem in fuel injection systems because such systems use no carburetor - fuel is vaporized by other means.
In the float type carburetor, the fuel is evaporated and vaporized immediately downstream from the throttle valve at the narrowest portion of the carburetor venturi. The effect of the evaporating fuel and decreasing air pressure causes a sharp drop in temperature within the carburetor venturi. In moist air or high relative humidities, the moisture in the air entering the carburetor condenses and, because of the lowered temperature, may result in the formation of ice. When ice forms in the carburetor, it tends to choke off the flow of air and reduce the power output, or even prevent the engine from operating.
Carburetor ice can be prevented or eliminated by raising the temperature of the air entering the carburetor venturi through the use of a heating device controlled by the pilot. The heat is usually obtained through a control valve that can be adjusted to allow heated air from the engine compartment to enter the carburetor. The pilot should apply carburetor heat whenever conditions are conducive to icing and in the manner recommended by the airplane manufacturer.