Hi Pierre,
Due to Venturi effect and fuel vaporisation carb icing can occur even at +2O°C depending on the relative humidity, no need for the outside temperature to be under 0°C.
On C150, 152, ... During the run up checks when the carb heater is On then hot air arrives into the carburetor which causes a drop of rpm (hot air is less dense than cold air) , this shows that the carb heater device works properly. In a real C150 the carb heater is always used like an on/off switch even if it is wired controlled
Does it help you ?
