Dan,

Air temps below 40 degrees F can certainly cause engine running problems! Glow fuel won't atomize as well, and the cylinder can easily get over-cooled. With normal running, we've seen cylinder head temps of around 350 degrees on aircraft engines. If you can hold your hand on it, it's definitely being over-cooled.

You should not have to mess with the idle mixture much at all, but your high-speed needle will need richening to accomodate the denser air at low temps.

It's also a good idea to go to a fuel with all-synthetic oil because castor can easily freeze in cold temps. It also becomes MUCH harder to clean off.

When flying in winter, getting the engine started is the main problem. Once it's running, it should be OK. If you continue to have problems, you can try small strips of aluminum foil to block part or all of the cylinder fins to see if that helps. That's just what full-size vehicles to in winter. They're 'tuning' the cooling system for cold weather operation.