i've sheared keys and i promise it's not going to affect your shifting, easiest way is to take the flywheel off and look, sheared keys can do some funky stuff, might be the case of partially sheared, then it'll start like sh*t but run like a mother f***er, i've had that with like an extra 8 or so degree's i figured, prolly wouldn't have lasted long haha, if you backfiring and popping do a plug check sounds to me like a fueling issue, clean that carb and check your jetting once