Airlocked oil pump: how to bleed

[ned from PA]

I accidentally tipped over my bike last night.  It kept running on its side (quite well, in fact), while I scrambled to lift it back up promptly.  I didn't have the presence of mind to fumble for the kill switch (in the dark) while the bike was on its side.  Once righted, it's oil light was on.  A blip of the throttle did not clear the fault, so I shut it down.  Two other attempts to get oil pressure failed, so I retreated to the house and scoured the internet for advice.

Nothing came up in this forum, but I found useful discussions by Kawasaki EX500 owners.  Apparently that bike is fairly prone to having its oil pump lose its prime.

What happened in my case is that the oil pickup tube sucked air into the oil pump, which then recycled this air bubble within itself.  Insufficient negative gage pressure on the inlet side prevented the pump from self-priming.

The solution is to bleed air from the outlet side of the pump, just like you do with an air-locked gear pump in a fuel-oil furnace system.  But there is no bleeder valve in a motorcycle's oil system. 

What you need to do is loosen the oil filter cover nuts a couple of turns, so that oil begins to dribble out.  That lowers the outlet pressure to atmospheric.  Then start the bike (with the cover still loosened).  This tactic permits the pump to develop a more negative gage pressure at its inlet, so it can finally self-prime.  Once that happens, you are off to the races.  Tighten the nuts, top off the oil, and you're done.

It's a permanent fix, because an oil-wetted gear pump no longer suffers backflow and will keep its prime.

I rode to work today, and the bike ran like a champ. 

This fix is simple enough that it could be performed while venturing far afield, so long as you have a 10-mm wrench.  Oil loss is quite minimal (a cupful or less), so you could ride home without topping off.  For bikes with screw-on oil filters, you just unscrew the filter a tad and then re-tighten.

Ned