I just recently rebuilt the top end of my 04 Yfz 450. I had to send it off to be shimmed cause when we got the head it didn't have the shims or the valve caps. Well now that we have it all together it will not crank. We have it perfectly timed and everything. We even advanced the intake cam and that helped it to crank a little as it fired up and cut right back off. We have fooled around and retarded and advanced the cams in all different kinds of ways to try to get it to run but we still can not. So now it's set back to the right timing and it will not crank still. Carb is getting gas so that is not the problem. Would you all agree that the shimming is not right? That's all I can thank of. I would appreciate any advice!