High rel. humidity helps too, my Aviation kindergarten days of trying to get over a hill in Chonburi Thailand, 2 up in a Rotax 503 (2 stroke twin) powered microlight, 10k h/w (going backwards) full throttle for about 3 mins before I decided it was useless, the little Rotax started to cough and wheeze, I cycled the throttle a few times and headed back, landed and immediately looked at the exposed engine and carbs to see the remains of ice melting off the venturi area, just downstream of the throttle plates, ice cold to the touch too, and it was a humid, clear, 34 degree day in the tropics, lesson learned.