got a new jug and piston kit.bored 68mm and installed it.the bike ran for about 5 mins and quit now it back fires and sounds like a 22 rifle.i took the fly wheel off and the woodruff key is still in place it acts like its trying to fire when the piston is on its way down by the exaust port.the reeds are still in good shape.i have kens milled head on.any ideas?maybe i need more octane because the compression is higher?