On GA aircraft, the ram air entering the ducting and surrounding the exhaust will stay quite warm, even after engine shutdown, If the engine failed in flight, by selecting carb heat, then attempting a restart, the ice will almost certainly melt and allow a restart. The "ice" we are talking about is more of a frost that covers fuel discharge ports in a carburetor, and will readily dissipate when heat is applied. The only likely time after an engine failure the carb heat wouldn't work is if you were in known ice conditions.