George, I'd guess your need to open up the main jet in winter has less to do with the fuel you're burning and more to do with air temperature. Old tractors seldom have heated air intakes (relying instead on one-piece intake/exhaust manifolds), so in the winter the air is going to be more dense. Pilots call this effect "density altitude", and most pilots know to lean the mixture on takeoff if it's a hot day.
The other effect of cold weather is incomplete fuel vaporization. The main difference between summer and winter fuels is, as Buzzman said, higher allowable vapor pressure in winter. In the old days before fuel injection this was done to improve cold-weather starting and to prevent vapor lock in summer. Now it's mainly to reduce summer evaporative emissions while allowing the use cheaper distillates (such as butane) in winter. But chances are you're still running summer fuel in winter, and in many areas there's not a big difference between summer and winter fuels. Anyway, assuming you use fuel with the same vapor pressure year-round, vaporization is going to be better in summer than winter, meaning you need a richer winter mixture.
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Today's Featured Article - A Belt Pulley? Really Doing Something? - by Chris Pratt. Belt Pulleys! Most of us conjure up a picture of a massive thresher with a wide belt lazily arching to a tractor 35 feet away throwing a cloud of dust, straw and grain, and while nostalgic, not too practical a method of using our tractors. While this may have been the bread and butter of the belt work in the past (since this is what made the money on many farms), the smaller tasks may have been and still can be its real claim to fame. The thresher would bring in the harvest (and income) once a y
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