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 Brief History of Tractors in Australia - by Bob Kavanagh. After Captain Cook's exploration of the east coast in 1770 the British Government decided to establish a penal colony in Australia. The first fleet arrived in 1788 and consisted mainly of convicts who were poorly equipped and new little of farming techniques. The colony remained far from self-supporting and it was not until the early 1800's that things started to improve. Free settlers started to arrive, they followed the explorers across the mountains and where land was suitable set up farms. T
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