OK, So I'm rebuilding my Brother's Girlfriend's Banshee. It refused to start, so I took it apart and it threw a circlip and the wristpin ate a huge groove in the cylinder, so we got a new cylinder and had both bored out to 65 mm. I got everything back together and it fired on first kick. I started it on six occassions after that for the next day with the air cleaner off and it started first kick. I then rode it around on my street with the air cleaner off and it ran pretty damn good although it was a little rich down low. I went to put everything back together and when I put the semi-dirty air cleaner back on it started first kick and within 3 seconds started dying. I gave it gas and it started to rev higher then it started popping and backfiring. It would not start after that and it consistantly backfires while trying to start it. This is with or without the air filter again. I ripped the whole bike down and took the head off again, the piston rings are still seated, I retightened everything down hooked up ALL carburetor connections securely and still no start. I have tried gas in the cylinder, starting fluid and nothing, we tried towing it behind my SUV and at about 17 mph it tries starting but just backfires the whole time. I have been working on and building race motocross bikes for the last 13 years and I have never EVER seen anything like this before from merely putting an air filter on. The only thing that concerns me is when you put the carbs on the clamps tighten all the way down, meaning I can only get them as tight as that, will new clamps help this or do I need new clamps and new boots? Could that even be the problem? I also replaced the carburetor connectiing tube with a brand new one, just to make sure that was not it. This has been going on into a 2 week project and it's really starting to upset me, Please Help!