RE: ys f-120 (messy?)
There are no crankcase vents on YS engines. Are you sure your plumbing system is not leaking. Fuel tanks for all YS engines are pressurized when running and for a considerable period after stopping. If you are using a conventional bung type fuel tank make absolutely sure that the seal is tight. Hayes 'O' ring type tanks are a better bet for YS engines. Also its best to wrap the tanks with a filament tape to minimise bulging.
John