The carburetor discharges liquid fuel into the air stream in the form of an atomized spray which evaporates readily. The heat required to evaporate the gasoline is drawn from the entering air, thereby lowering its temperature. The cooler air chills the interior of the carburetor and may cause the moisture in the air to condense into droplets.
Under certain conditions of atmospheric temperature and humidity, the liberated moisture actually collects and freezes on the chilled carburetor surfaces, especially on the throttle plate and surrounding throttle body. When the throttle is almost completely closed for idling, this ice tends to bridge the gap between the throttle plate and throttle body, thereby cutting off the air supply and causing the engine to stall. Opening the throttle for restarting breaks the ice bridge but does not eliminate the possibility of further stalling until the engine and carburetor has warmed up.
For carburetor icing to occur, the outside air must be cool enough so that the refrigerating effect of fuel evaporation in the carburetor will lower the temperatures of the throttle plate and body below both the dew point of moist air and the freezing point of water. The air must also contain sufficient moisture for appreciable condensation of water to occur when it is chilled in the carburetor.
Generally speaking, carburetor icing occurs when winter grade gasoline (more volatile than summer grade) is used and when the atmospheric temperature ranges from 30° to 50° F. at relative humidity in excess of 65%.
Carburetor icing problems can be reduced by the use of anti-icing additives, such as alcohols, in the fuel. Some fuel refiners use anti-stalling additives in their gasolines which have proved effective in combating carburetor icing.
Another form of carburetor icing has been observed in some engines during high-speed driving on cool, moist days. When certain cars are driven steadily at 60 to 80 mph, the large quantities of cool air passing through the carburetor may result in gradual ice formation within the carburetor's venturi. Since this ice restricts the venturi passage, the resultant increased vacuum in the venturi tends to increase the rate of fuel flow. The fuel-air mixture thus becomes excessively rich, causing loss of power and high fuel consumption.