I think the the three ton trucks had a manual heat riser valve (at least on AD trucks). Heat on all winter, heat off for summer. Vapor locking on warm winter days! Here in the great white north, intake heat is a good thing. A few minutes after I start my truck, which has intake heat, my carb starts to freeze. Now, I don't think the intake manifold would freeze up, as there just isn't the restriction and drop in pressure throughout the intake, but I do get condensation on the outside of the intake, and if it's cold enough to condense water vapor, surely there would be drops of gas condensing inside there. I also notice, that my intake heat comes back on after the truck sits at idle for a few minutes. I have also noticed, that on days when its fairly cold, like 20 farenhieght, the heat never shuts off. That's my non-scientific two cents.
216 Carb Freezing Pic
http://i221.photobucket.com/albums/dd45/HomeCrafter/IMG00014.jpg
