Help! Oil issues

[Bristolbadger]

Alright guys, I need some expert help again

After a joyous few trouble free months on the old 750 bob - I finally got round to syncing the carbs and with the new points and reg/rec she's been running so sweeet... however it ended last wk after a evening ride resulted in the RAC and flatbed Greg to recover me home  

15 mins into a decent duel carriageway blast I stopped at some lights, lost power and died then oil puked out from under the oil tank cap. (Not the breather)
I moped it up checked the level and still seemed good, cooled down for 10 then she fired up with no probs so I gingerly carried on and all was good, no exhaust smoke or odd engine noises. 3 miles later I stopped to check things over and the oil upwelled/puked again and had been leaking slightly.

After some thought I guessed I might have over filled as I'm running a custom tank, 2.2L but the level was good.
Next day I fired her up again and after 10mins of idling and seeing oil returning to the tank a 1st gear pootle down the road I had another look in the oil tank cap and it welled up and overspilled again.

I dropped the oil and removed the oil pump and filter and had just over 2L - I lost a fair bit when it puked each time. No bits of anything in the sump pan or pump screen. I've since checked the pressure relief valve and it's all good, not stuck and piston moving freely, likewise with the oil stopper valve.
The filter was fairly grubby and needed a change.
The feed and return lines are clear and free flowing likewise the crank case hose and the head breather hose.

I'm a bit stumped, what else should I be looking into?

[Oddjob]

Not a 750 expert and never professed to be but I'd be checking crankcase compression, has the vent pipe been blocked or kinked. Run it detached and see if it does it again.

[Bryanj]

Sounds to me like you topped it up when it had been stood a while and oil tank had drained into sump

[Johnny4428]

Yes as Bryan says, have checked mine after short run in garage and only half way on dipstick check another time and up to full mark without any topping up.

[Bristolbadger]

Not a 750 expert and never professed to be but I'd be checking crankcase compression, has the vent pipe been blocked or kinked. Run it detached and see if it does it again.

No kinks or restrictions that I can see and firing on all four with power until she pukes and stumbles.. will put everything back together with fresh oil etc and see how she goes

[philward]

You mentioned you are running a custom oil tank, has it only happened since you fitted the custom tank? Just thinking whether tank internals affecting anythink

[Bristolbadger]

You mentioned you are running a custom oil tank, has it only happened since you fitted the custom tank? Just thinking whether tank internals affecting anythink

No the tank isn't a new addition and not had any issues with it in the past. Nice and clean inside and no blockages to report

[K2-K6]

Possibly worth a run through of attributes in dry sump system fitted to those engine, maybe it'll prompt some further thoughts.

Scavenge pump should always return more volume than supply (else the system won't work) to keep sump area empty and all of volume available to the pressure side of pump to feed lubrication distribution.

Return pipe to tank should be above oil level on entry to tank. It can have air in return as it's just trying to suck sump clear when running.
Tank needs venting to avoid the return pressuring the contents, this usually has a air/oil separation to avoid oil being pushed out vent.
Could be vented to motor, but motor needs venting to avoid crankcase pressure building (they can't just constitute a closed system) usually this will be directed to air intake to avoid vent to atmosphere, or into a catch tank (also open to air) in case of oil being expelled.

The crankcase venting can't be dumped to oil tank (to use as a catch tank) it must be to atmosphere or into lower than atmosphere-ish, like an airbox to then ingest fumes into combustion.

Basically all the areas have to be effectively vented to atmosphere to avoid pressure building,  which is happening here. Somehow the tank etc is going over atmospheric, hence the oil just pumps out as soon as it gets a route that's unimpeded 

Too much oil for tank volume will cause a problem because the pump is trying to stash any available oil in there, but it should already have gone out the tank breather before you open the lid.

[JamesH]

Sounds to me like you topped it up when it had been stood a while and oil tank had drained into sump

I'm fairly convinced this is the issue if I were a gambling man..

[paul G]

Sounds to me like you topped it up when it had been stood a while and oil tank had drained into sump

I'm fairly convinced this is the issue if I were a gambling man..

When mine has been stood over winter I drain the sump first before starting it.
You would be amazed at the amount in there, just put it back in the tank.

[Johnny4428]

I did the same Paul! A lot of oil.