It isn't easy to tell at no load, since it is 4 or 8 stroking continuously, but there is an odd sound as if possibly one cylinder is hitting harder than the other. Does it have a 2 cylinder intake manifold on a single carburetor? Might be a mixture distribution problem, or leakage between sections of the crankcase. However if it cleans right up when you put a load on it, I may be misdiagnosing the whole thing.
