I agree with 'cuda. Those are symptoms of a rich low-end. The O.S. uses an air bleed, I believe. The air hole in the carb should be about half-open. If it's closed or nearly closed, back out the screw until it's about half-way open. Then back out the high-speed needle about three turns, and start from there. If the O.S. doesn't use an air-bleed, never mind.