First off its not fuel. Its excess oil. What kind of nitro are you using? The first 5 tanks should be run rich and it is normal to see oil dripping out of teh exhaust. You also need to watch yuor temps expecially durring break in. Durring break in it is a must to get temps above 200. Not much hotter than that either. Once broke in you can get your temps up to around 250-270 for best results.