Hello,
My atv won't start and has air blow out from intake to carb. I have removed the cilynder head and poured water from both valve sides and there were no leaks, I have lined up the tdc on piston l then put the timing (valve) gear to the notch on the cilynder head. So what could possibly be wrong?

 

Cam timing is fairly "wild" on quads, and being a single there are no other cylinders sucking air in, so they tend to spit a mist out of the bellmouth as they run. You don't notice this until you have a problem and take the airbox off. Look elsewhere for the fault, partly blocked carb, no spark etc.