Last fall, at our last Fun-fly of the season, it was pretty cold at about 35 degrees with a stiff breeze, and a lot of people had trouble geting engines started, myself included. I found that I had to richen the high speed needle nearly a full turn to get it running properly.

Cooler air is more dense than warm air and can throw off the tuning, especially if the temp difference since you last flew is 20 degrees or more. Try richening the needle. (But I agree, 50 degrees shouldn't cause hard stating.)
Dennis-