Colder air = more power because you get more fuel and air into the motor . When the air is cold and dense you get more air into the motor thus making it run lean... to compensate richen the high speed needle 1/8 at a time untill you get it where you want it. Hot air of corse is the opposite. Humidity also has something to do with it... the more humid it is the richer these motors run due to the extra water vapor in the air.