Double and Triple check your timing. I rebuilt my YZ450 and swore I had the timing right. Bike wouldn't start. Checked and double checked and turned out I was off by 1 tooth. Fixed that and the bike started 1st kick. I'd be careful in bumpstarting the bike since there is certainly a problem somewhere. These bikes should start with the kicker pretty easily when properly set-up. If the timing is off you can do some damage for sure.