why would she periodically start backfiring under high compression situations? like going down a steep hill in second gear. when not backfiring it doesn't have that sweet hum, it blubbers or burbles. It only seems to do it once it gets to normal operating temp. then maybe after a couple of weeks stop and not do it again for maybe another six months. I take it in and of course it won't do it. otherwise it runs good. no problem accelerating. []