Magnum 2 strokes carbs tend to have a problem with air leaking around the high speed needle. If you can correct this problem they run fine but Magnum needs to address this. I did have a XLS 91 that would not run correct with a pitts muffler; why I do not know. If you used the stock muffler it run OK but was hard to keep tuned. Bottom line YOU GET WHAT YOU PAY FOR.