Carburetor icing occurs when there is humid air, and the temperature drop in the venturi causes the water vapor to freeze. The venturi effect can drop the ambient air temperature by 30-40 degrees F (16.7-22.2 degrees C), therefore carburetor icing often occurs when the outside air temperature is in the 60-70 degree F (15.6-21.1 degree C) range. Unfortunately, the warm air temperature often causes pilots of aircraft to overlook the possibility of carb icing. The ice will form on the surfaces of the carburetor throat, further restricting it. This may increase the Venturi effect initially, but eventually restricts airflow, perhaps even causing a complete blockage of air to the carburetor. The engine begins to run more rich as ice formation increases. Without intervention (carb heat or leaning) this can only continue until the mixture is outside of the "chemically correct" range for combustion. Icing may also cause jamming of the mechanical parts of the carburetor, such as the throttle, typically a butterfly valve.
While it applies to all carburetors, carburetor icing is of particular concern in association with piston-powered aircraft, especially small, single-engine, light aircraft. Aircraft powered by carbureted engines are equipped with carburetor heat systems to overcome the icing problem. In cars, carburetor icing can occasionally be a nuisance. The inlet manifold and parts of the carburetor often have warm water from the cooling system or exhaust gas circulating through them to combat this problem. Motorcycles can also suffer from carburetor icing, although some engine designs are more susceptible to it than others. Air-cooled engines may be more prone to icing, due to the absence of warm coolant circulating through the engine.
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