jerky

I noticed yesterday that my cable on the start is jerky and I have to pull it insanely hard And quick to get it running, if I don't pull it hard or quickly enough it is jerky and it get locked, kind of like a seatbelt. Does anyone know why this is the problem?

Any help would be appreciated... I put a 4.0 hp Subaru Robin on one of my minis. However, it shakes really bad under accelleration. It's not the engine, it's running really well. I'm thinking it's the chain tension? How tight/loose should it be? Could the clutch be going?