I know this has been a while, but if anyone else need this info. I had a similar problem with one of our 86 KLF 300A. I could not get it to start. It would fire then sputter or backfire. I took carb apart and cleaned and adjusted valves and put in fresh gas and spark plug. I was able to get it running but it was not running good and would backfire. After running I saw smoke coming from behind battery. What I found was a chafed ground wire grounding out on frame. Tracing the wire it went into left engine cover. I assume the pickup coil was not grounding properly. Fixed wire and now it runs great.