The ice in the carb is mostly from the air that is pulled in with the fuel. When the fuel sprays into the air, the evaporation is responsible for a big temperature drop, as is the pressure drop when the air is pulled through a nearly closed throttle. The combination supercools the airborne moisture, which then sticks to the walls of the carb. The frozen mixture is ice and gasoline mixed, so it will burn (sort of).
The danger is from carb throat restriction (thanks Whirly!) and also that the newly formed carb throat is ragged and makes for turbulent flow of the air, thus bad for mixing and distributing the air-gas mixture. The rough running and rpm drop are a product of the lost efficiency.
Of course, if the engine is tightly governed, it will not drop in rpm as much, but the slight roughness might be noticable, anyway.
The worst times for carb ice are in low power descent into moist air, where the cool carb and throat are just waiting for that moist air to deposit itself on the walls of the carb.
Last edited by NickLappos; 2nd Mar 2005 at 08:36.