Your real problem is most likely that you are running that engine too lean. The fuel/air mixture in a glow engine controls the engine's ignition timing (along with compression ratio and the "hottness" of the glow plug). When the mixture is too lean, it fires too early which causes the engine to get hot which in turn causes the engine to fire even earlier until it causes the prop to loosen. Get a tachometer and use it to tune your engine. Adjust the needle valve for maximum RPM and then open the needle valve to lower the RPM by 500.