A condition where ice forms at the edge of the carburetor throttle plate/butterfly . It restricts the flow of the fuel-air mixture when the throttle butterfly is at or near the idle position. It causes the engine to stall. Ice forms because of rapid vaporization of the fuel lowers the temperature of the mixture and causes the moisture in the air to freeze. It can occur when the temperature is between -2 to 13°C (28 to 55° F) and the relative humidity is above 64%. It usually occurs after the engine is started and before it has reached operating temperature. Also see carburetor icing .