This is how it sounds when piston hits the valves. The sound is metallic, almost ringing at higher RPM. On this particular 49cc scooter engine exhaust valve hits the piston. This happened after crankshaft and camshaft replace due to crankshaft failure. Remeber to check the clearance between piston and valves with clay or play-doh after swaping this componets (or cylinder/piston/head). Whats interesting when I turned the engine by hand or by electric starter there was no such noise. This can be solved by enlarging notches on the piston or adding another gasket depending how much clearence we need to make.
