I have an 07 yz450f I have recently installed a new head and timing chain. I got everything back together and kicked this thing over a bazillion times and it will not fire. I've searched this site and researched all the threads. What I came up with is to check fuel, timing marks, count chain pins, make sure shims are seated and correct clearance. I've tore this thing apart 3x. I have spark, per taking the cap off and kicking it over... It sparks! I've checked timing, flywheel is lined up with the "I" the cams are line up at 3 & 9 on the gasket surface. I have 13 pins between cam 12:00 marks... Not sure how that would ever change. Can somebody give me a direction to look towards, I'm at a loss!