wet sumping '74

Did not want to hijack Mark's topic.
Friend's 850 wet sumps before his eyes. Oil tank we are using has a sight tube and you can see the oil level falling after a run, however, it stops leaving about a quart in the tank. Why would it wet sump then stop? Crank it up and the oil tank fills up to 3/4 on the sight tube.
Thanks for your thoughts.
JT

:shock: If it was an internal leak or damaged pump, I don't see why it would stop. Could it be a function of the non-standard oil tank ? Is it siphoning somewhere (perhaps sending the oil back down the breather ?) or is there a vacuum forming which is drawing the oil out of the sight tube but not out of the tank ? It's difficult to say without seeing the tank.

I would be inclined to remove the sump drain. and check if the oil is actually pouring into the crankcase at that rate. It must be going somewhere.

79x100, the tank is alloy, based on the central "S" tank. Breather arangement includes the brake booster valve recommended on this site and others, and a line out from a baffled breather. This is not an oil usage issue, as on restart the oil returns to the original level on the sight tube. Will check, as you suggested, the volume in the crankcase.
Thank you.

How many miles since the oil pump was lapped in?
Oil generally finds its way from the tank to the sump due to slack pump gear clearances (that can be removed by lapping which should be considered a 'service' job) ensuring the pistons are left near TDC (big ends held at highest position) can slow the draining process, and multigrade will drain faster than monograde oil.

However, there are other things that can cause (or add to) the oil to drain down quickly:

Leak at pump gasket face.
Timing cover gasket (damaged at relief valve drilling).
Jammed oil pressure release valve (not 850 MkIII model) feed oil will reverse flow through the valve.

L.A.B., the machine has 10K miles, but has been inoperative for over ten years. We rebuilt the top end and she has about 200 miles since. The pressure release valve was cleaned after we obtained only minimal oil pressure on 30 wt. non detergent oil. We now see 30 lbs at start-up with one shim in the valve body.
Would pump work be consistant with ten thousand miles on the clocks?
Thanks for your help.

A check of the oil pump clearances is probably worth doing and 10,000 miles seems like a good service interval to do it I think.

30 psi with cold oil on start up seems rather low! As the pressure release valve should normally be set to operate between 45-55 psi.

Unless the motorcycle is being operated in temperatures below 0°C (32°C) the general recommendation is for SAE40 (or 20W/50) oil, or SAE50 above 32°C (90°F) although SAE50 can be used for general summer use.

http://www.norvilmotorcycle.co.uk/techtalk27.htm

These oil recommendations I expect apply to UK seasonal temperatures, so this would need to be taken into account.

Thanks 79x100 and L.A.B., will let you know what I find out.
jt