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Highflyer is right about the carb heat bypassing the airfilter. Another use of the carb heat is if the airfilter should become clogged. On fuel injected engines they have alternate air in case of a clogged filter.
A slight loss of RPM is the first indication of carb ice. If you notice an RPM drop then try the carb heat. If ice is present then the engine will run rough shortly after applying the heat. As soon as the ice is melted, the engine will smooth out again.
Most icing will occur between 20 and 70 degrees F. But it can occur as low as 0 degrees F or as high as 100 F. After you have flown in a certain plane awhile you will be able to detect even slight changes in the engine.
Of course as soon as the sun goes down or you are over water and out of sight of land the engine always sounds rough.
Mike S.
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