It's the YFZ450 engine. Same design as the WR, YZ as far as the oiling system goes, so the question applies here.
Rebuilt the engine from bottom up. Was careful to ensure all o-rings were in the proper place. Oil pump was not replaced and was within specs according to the service manual. It appeared fine. Dumped 1 quart into the crankcase, 3/4qt into the resevoir, and 1/4 into both the valve cover breather tube and the oil filter chamber with bike leaned over. Let it sit for 30 minutes and cranked it over for 30 seconds with plug removed and get nothing at the head check bolt hole (removed completely). Pulled lower right oil line into engine (below clutch) and put finger on the hole and feel no vacuum pressure during cranking.
Any tips? Is it possible the oil pump is bad while appearing to be in perfect condition?
Edit: Couldn't find the answer anywhere, so I figured I'd come up with my own original thought. Pulled the resevoir vent tube that goes to the valve cover and put 20 PSI to it with plenty of oil in the resevoir. Immediately heard air escaping from pressure check bolt. Kept it on there for 10 more seconds and oil started spewing. Hit the starter with spark plug out and oil began flowing.
Beats me... my WR250F gave me no trouble.
Edited by 86bg, April 23, 2013 - 02:33 PM.