Did the engine used to run fine, and now it's dogging you? If it's never run well, or you bought it secondhand after someone overhauled it, one thing a buddy and I ran into is an improperly inserted cylinder sleeve. This was with an OS .55; we were at our wits end getting it to run, and we're both experienced with nitro engines. We sent it back to hobby services and that was the problem: the cylinder liner had been inserted 90 degrees off. The engine would start, run, but not really respond to throttle nor any needle valve adjustments. Take a look in the exhaust port to see if it looks partially blocked. That might indicate a misaligned cylinder sleeve.