Yeah, that exhaust timing is a little low.

Two important things on these little engines are to use a free flowing exhaust; (I use two 1/2" i.d. outlet tubes and NO baffles in the muffler), and a larger carb. 11mm is a good size carb venturi for these size engines. I don't recommend any porting or other mods until these two things are addressed first, assuming of course that the engine is in good shape to begin with and doesn't need rebuilding first....

AV8TOR