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 - The River Tractor - by Charlie English Jr.. This is a 4010 John Deere that was used to farm on the Diamond Island which is in the Ohio River about 15 miles southwest of Evansville, Indiana. About ten years ago, this tractor was parked close to the bank as the river was washing the bank of the island away.
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