Troyp: The engine surges because it revs up and burns all the fuel in the crankcase. Then when it sags the fuel has time to build up in the case until it picks up and burns it out starting the cycle all over again. You can leave the heater on and run a really rich engine, but a lean one won't run without fuel. Better to start a little rich and set it than to try to set the needle exactly. I always open my engine a few clicks then lean it out. This is just my starting routine. Some engines will keep a setting for a long time. I've read about people who set it in the spring and don't change it for months. Also unless a plane is really underpowered, I don't lean it out trying to get the last few rpm out of it. This gives me some room for the mixture to vary a little without making any difference in performance and avoids a lean run.