yz450f timing mark

have a 2003 yz450f and I am currently putting it back together from a blown head gasket, I have the head on and I am now trying to get tdc by lining the line up with the engine cover and as it's about to line up it notches over and will not line up, as it gets to tdc it gets slightly tight then falls past its mark, has anybody had this trouble before, what can be done. Thanks

If you are rotating the engine with the starter, you'll never get it to stop in the right place.  Put a wrench on the crank and do it that way.  Asa you turn the engine with the spark plug in place, it compresses air in the cylinder.  That pressure will bleed past the rings if you give it a second, then the crank will stay at TDC.  Remember that the crank can turn 3-4 degrees either way and barely move the piston, so it's easy to rotate it off TDC as you work.

Yeh mate i have been turning it over with a wrench on the crank, but I have the spark plug out, should I have it in

With no plug, there should be no tendency for it to roll over the top unless you have the cam lobes pointed down, which would be incorrect.

Well I have no spark plug and no cams in, just the head on and when I crank it to tdc wig a wrench it pops past the tdc mark every time.

If you feel ANY resistance going over TDC without the plug in or the cams in place, something is wrong. 

What grayracer said. Or is not so much resistance or a tendency to overshoot TDC. Is it possible the flywheel weight momentum keeps it moving? - I suppose not.  Have you also tested for TDC with a probe through the plug hole? I preferred to do it that way and turn the motor by having it in top gear and nudging the wheel round - not an option until the motor is back in naturally ;)

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