Intake blows exhaust sucks

Yamaha

I have a YFZ450 that wont start. When you hold your hand over the intake you can feel it blowing, when you put your hand over the exhaust it sucks. So here is what I have done so far. I have checked the timing and rotated it 180 and checked the valve clearance and adjusted it. After I redo the timing and crank it by hand its fine it will suck through the intake and blow through the exhaust. But when I crank the bike it switches back to blowing through the intake after a few seconds of cranking. What am I missing?

what position was the piston in when u set the timing.

TDC. 3rd hash on the flywheel.

Found the problem. The gear on the intake cam had spun, so the timing marks were on but it was way out of time. Working great now.

Did you figure out why the gear spun in the first place? Odd, I don't think I ever heard of that happening to a YFZ450.

Glad you got it going again.

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