I don't have a .21 engine at the present time but from my expereance nitro engines range from 240f to about 270f depending on the engine and the tuning. If your engine is tuned coeectly you should be running at about 220f after a one minute run. This should go up the longer the engine is run until you get up to about 250F to 270F. I have two .12 nova engines and they run at about 260 to 270F. You should tune for performance not temperature. If you are getting good power at the high and low end, I would not worry about it.