I recently took apart the top end of my 2006 yzf450 to change the shims. After I finished and put it back following every guide I could find telling me to put it to tdc etc. I have done everything exactly the way I should but the intake camshaft does not line up with the head like its supposed to. Now it won't start and it keeps backfiring on me. Yes specs are good, it's at tdc, shims are sitting correctly, chain is on properly everything is done the way it should be. What could be the problem ???