So I recently blew my motor up on my 1998 Yamaha warrior, I then fully rebuilt a top and bottom end off of my other 2002 warrior and threw that on there. Now I have spark, good compression ( put my finger over the hole and it blew it off ) and good fuel. And the battery is charged so I have no clue what the problem is. It will start for half a second and continue to spin.

 

Double check that the timing is right, both TDC by the flywheel actually is TDC by the piston, and that the cam timing agrees with the TDC mark. If they are OK, change the plug and try again. If it still won't start, clean the carb out, or just swap to the spare carb and see if that fixes it.