Thats very strange. It seems totally backwards. i assume you turn off the petcock each time you park? Seems like one of the carb circuits (idle?) is plugged when hot.
I wish I had your weather, but I am glad I do not need a disc lock, I am 15 miles from the border of Detroit and I leave my key in my ignition ll the time. My parents (lived in the same town I do now) didn't even have keys to the house when they sold it, never locked the doors