Jerry (Hot Rod Lincoln) has an excellent explanation in his post.
Carb icing is very common on older inline 4 and 6 cylinder engines. I have only once personally experienced icing on a V-8, but that was a Holley carb which explains this unusual occurance. Carb icing normally occurs between 25 and 40 degree F. when the relative humidity is quite high (rain, fog, snow, etc.) and generally at highway speeds. If icing is experienced, normally pulling to the side of the road, and running the engine at a fast idle for 30 seconds or so will clear the icing.
The reason for icing at highway speeds is twofold: (1) there is more fuel going through the carburetor, thus more heat (called the heat of vaporization) is required to vaporize the fuel and (2) at the higher speeds, there is more air circulating under the hood removing the engine heat at a greater rate.
Carburetor tune also plays a major factor. A carburetor with an overrich mixture is more prone to ice (which explains why my Holley, notorious for running too rich, iced on a V-8).
For several years we have suggested that owners of older vehicles could acquire an air cleaner from one of the early 1970's Toyotas or Datsuns which had a snorkel with a flip switch to control fresh or heated air. Hook this switch up to a choke cable, and the heat could be controlled from the driver's seat.
Jon.
