MDH seems to have hit the nail on the head, it is more than likely the resonance of the whole system that is causing the problems. Whilst I have many years experience of 2 stroke exhaust design, I cannot understand why the exhaust on the cars I have looked at is so firmly attached, it needs some flexibility to dampen out the resonance/harmonics, it could be just the rubber mounts that need modifying and their positioning.On a lot of our race bikes the entire exhaust system is held on with just a couple of springs and rarely do we have any problems.
