Originally Posted by Greg58
Sounds like a air leak on the carb or the intake to cylinder, I would check everything I could for loosness for the vibration.
Ditto: Your symptoms are consistant with an air (vacuum) leak. Most likely at the carburetor/intake manifold or the intake manifold gasket at the cylinder.
You didn't say what carburetor you have. That might help us make some recommendations. If you can assure that there are no leaks at the intake system then you could be looking at a bad crankshaft seal either on the clutch or magneto side but check that intake area first.