Was at a dealer today and was talking about all the stuff we talk about on here and he showed me book that had updates. The carb heaters should be off if colder than 30. If warmer than 30 then turn them on. It sounds backwards because the potential for freezing has to do with humidity. When it's colder than 30, there is little humidity and is less likely to freeze. Over 30 the humidity is high and could freeze. I read it right out of his book that was published for DOO techs.