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My impression is that very few people use carb heat in carbureted Rotax 912 engines. The installation manual for the carbureted 912 series basically states the factory air box has provision to accept preheated air but no means of supply is designed; so one is on their own to design a means to provide preheated air to the air box. There are a couple of minor (well for a Rotax engine manual anyway) notices about icing being a potential problem but not much else. Are you certain it was icing?
Herb, are your carb bowl vents enclosed under the cowling (not exposed to ram air)?