RE: Info on .91FX
With many engines over .90-size or so, there is a "knocking" sound. We've experienced it on a number of engines in a number of brands. This affects 2-stroke engines of larger displacement. Usually it's heard in low- to mid-range running. We have performed repeated tear-downs of engines and have never seen evidence of metal-to-metal contact that could make such a noise. Our only conclusion is that the sound is part of the exhaust note and is made by unburned fuel continuing to burn after it leaves the exhaust port.