Can I just say that when going to carb heat to check for icing, leave it that way for some time - twenty to thirty seconds is a good minimum time.
Sigh! ....
biscuit74 the idea is to CHECK for icing ... it takes a few seconds: Pull out the carb heat control - observe an instant rpm drop with NO other indications - push in the carb heat control - observe an instant rpm return to cruise setting. Now you KNOW there is no carb icing and can get on with flying the aeroplane, not faffing about with head in cockpit.