Brett - Really depends on how cold you are flying in. The picture I uploaded was my last flight of the season here in the Yukon, and it was about +3 deg C. I am using 10% and really nothing special to prep for that. When it gets about 25 below zero Cent, the engines become hard to start and will only run at full throttle. They have to be warm and you have to be quick to get them going.

I have read of where you can use a can, like pepsi, cut it open and wrap the top of the cylinder and then use a hose clamp to keep the engine from loosing too much heat. You do have to watch that the enigne does not overheat.

I was also going to try aluminum tape to help keep the heat in as well.

The only thing is that after you get it going, the wind from the prop will freeze your hand while you adjust the needle for the flight. And if you wear glasses..............
Well I almost lost my viper on that day as well. Made a high speed pass and took a deep breath, my glasses fogged up and I lost sight of my plane for only a few seconds. Cold weather acts like a turbocharger and the Viper was really ripping. Took us a while to find it and it was a really wild experinece flying a RC plane for about 15 seconds without seeing it. Kinda like blind flying. My flying buddy spotted it halfway to the mountains in the background of the picture, heading straight down[X(], we did get the plane back for a safe landing, and with the addition of anti fog on my glasses made two more flights that day!