Oil pump not priming on fresh rebuild


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

You missed a point about the oiling system, and that's where your confusion started. The WR, like the YZ450, is a dry sump oiling system. In a dry sump, oil is stored apart from the actual crankcase, and pumped from there to all the places it needs to go. Then, as it drains down to the crankcase sump, it is sucked up and returned to the "tank". In the late WR/YZ450's the "tank" was moved from the frame down tube to a cavity in the front of the engine.

In order to show any oil circulation on the feed side immediately after an oil change or fill, the oil would need to be poured into the feed reservoir, or tank. One of the things about the current design that has always struck me as kind of stupid is that when you refill the oil, you put it not into the feed reservoir, but into the crankcase. Because of that, when the engine starts, there is no oil feed until at least some oil has been picked up and "returned" to the oil tank to supply the feed pump. That could not have happened in the 30 seconds of cranking you gave it.

I put 1qt into each the crankcase and the resevoir. I cranked it for 30 seconds on 3 different occasions with the starter, then 40 seconds more with a drill to no avail. I guess the oil pump was "airlocked." I've seen the problem with water pumps before. The air compressor trick solved it immediately. Holding my finger over the hole where the resevoir is attached to the engine for oil input into the oil pump showed no vacuum pressure at all

Edited by 86bg

Even if the feed oil reservoir, wherever it happens to be, is partly or completely filled after a rebuild, there will be a column of air running most of the length of the oil passage (including any external tubing) that runs from the reservoir to the pump, and beyond the pump to the right crankcase cover, where there is a low pressure check ball. This is what prevents oil from migrating almost immediately to the crankcase from the "tank" while the engine is shut off. When the pump first starts turning under these conditions, it can take a while before it can generate enough air pressure to pop open the low pressure check and start moving oil. At cranking speeds, the pump will not do this as quickly as at idle, and a lot of people panic a little at that. If the engine was assembled with the correct prelubrication, there's not much too be concerned over; it can be started and will be fine until it clears out the air on its own. Your method of priming the system works, of course, and from a purist standpoint, it's better to do it than not to, but 99.9% of the time, it's not strictly necessary.

Ah, YES! The check ball valve. Thank you.

Im having the same problem but when i use my air compressor on the breather you hear a bubbleing sound in the engine, i took off the external tubing cleaned it. gave it a blast and some oil bubbled up but that was all that it would do i put the tubing back on and for another 10 minutes i was shooting air into the bike but no oil was coming up do you guys think i should try one of those brake pumps to suck the oil up then do the compressor again?

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