Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

You need to remove the steam  chest cover and check the  valve is moving equally .
Your timing can not be too bad as it runs
Is the surface between valve and ports smooth?
I would try holding the engine on it side (steam chest up ) to see what happens .
Is there any oil in there .
Does adding oil help ?if it does then I would be looking very closely at the surfaces .
 Another thought is air getting past the piston ?