Determining if your engine is worn out can be quite difficult, especially if you are new to the hobby. Common symptoms are that the engine will start and run for a bit, and stall, refusing to restart until the engine is cold again. This is because of the tapered design of the ABC sleeve in most engines. It will get enough compression to run when cold, but when it warms up and expands, the piston can't seal properly with the sleeve. The engine may also get impossible to tune, constantly overheating even when rich. This can all be frustrating and confusing to people who are new to the hobby.