Which O.S. .25? SF, FP, FX?... (several others...) And what problems are you having with it?

If possible, get in touch with some experienced modelers, who can help you learn safe starting proceedures. It'll save you a lot of cuts on yor fngers. (maybe save you from losing a finger...) It'll also help you to operate the engine efficiently and to get the best operating life you can from it. 9which can be over 20 years... I've got a .25 SF that's close to 20 years old now, and still running almost like new.)