I fly 2 aircraft that regularly will quickly build up carb ice on certain CAVOK Australian mornings while taxiing. Some Continental engines are well known for this. As aterpster mentions, certain power settings temps and appropriate relative humidity and you are potentially in the zone, even when temps are warm.
NB Not saying icing is the issue for the OP's particular issue but FWIW I regularly see students and long time pilots on engine runups going through the ritual of pulling the carb heat for a second, saying RPM drops, then immediately pushing it forward, without actually checking to see if they may have built up ice. The check should not just be to check the revs drop. It can be an eye opener for them when I get them to hold the carb heat on and they start to see the engine RPM increase and when they return to cold see the RPM now higher than their initial setting.
