Engine will idle fine but once it revs past about 1500rpm, it starts shuddering like it's running on one cylinder. It only has two cylinders. This happens when i'm on the plane then continues once i stop and rev it up again. If i hold it flat out in neutral, it will sometimes come good and i continue on my way for a while then this problem returns. Plugs are brand new. Had a similar problem with a Yamaha 25hp and the carbs was a bit dirty. Any ideas?