Carburetor icing will usually occur when the ambient temperature ranges between 30° to 60°F, when the relative humidity is above 65%, and when the wrong grade of gasoline is used.
The most distinguishing symptom of carburetor icing is that the engine will stall repeatedly during warm up. However, the engine will run normally when it is first started (cold) and after it reaches normal operating temperature.
The ice is formed from the condensation of moist air passing through the carburetor. The ice collects on the primary throttle valve and main body bore. Since the throttle valve may be nearly closed during engine warm up, the ice will bridge between the throttle valve and body bore, and seal off the engine air supply, causing it to stall. Opening the throttle valve during restarting will break the ice bridge.
Occasional quick opening of the throttle during warm up may help reduce icing. A hot-air duct system is installed on 1972 vehicles to help prevent carburetor icing and control emissions.