It is actually opposite of what you think. You need to richen the fuel in cold weather, don't treat the engine as a thermostat, but an engine. Why you are asking? Because cold air is more dense than hot air, thus, the engine is pulling in more oxygen in the cold than it would in the hot. So you need to feed it more fuel, if you do not richen the mixture, you are "leaning" the engine mixture out just by driving it in cold weather. And vice versa, you are "richening" the engine mixure when driving it in warm/hot weather.
Cold Weather - More oxygen, so you need more fuel (richen it)
Hot Weather - Less oxygen, so you need less fuel (lean it)
Pretty counter-intuivive huh?
Look at it from an air density point of view rather than an air temperature point of view. You are driving it in well below freezing weather, you need to richen it up, a lot probably. There should be even more smoke than normal, because it will need to be more rich than normal. Your engine is running at 350F, there is something wrong.
Another thing that could cause it would be poor air flow to the engine, you said you were wrapping it in tin foil? How much of it? That could be trapping in the heat that needs to get out? Or not letting enough air flow through the head in order to keep it cool.