Carburettor icing is caused by air passing through the venturi and over the throttle valve expanding and local temperature falling.
Heat is lost from the air due to the latent heat of vaporization of fuel, this takes place in the vicinity of the throttle valve, resulting in a local temperature drop up to 33°C (60°F.)
The cumulative effect of both expansion cooling and fuel vaporization reduces the temperature to the point where ice may form on the throttle and in the adjacent passages of the carburettor, even though the ambient temperature is well above freezing.
Each type of engine/installation has particular icing characteristics and generally the lower the rpm, the more likely ice is to form due to a combination of factors. For example, the location of the carburettor in the engine bay may mean that it is kept warmer by a full power or cruise power engine setting and is thus less likey to suffer icing than at lower rpm.
However, this is not a universal rule.
Last edited by Final 3 Greens; 17th Oct 2004 at 04:14.