Crankcase filling with oil

[A7restore]

Hi. I have just tried starting my A7 plunger engine for the very first time after a complete rebuild. The engine starts and runs, however the crank case fills with oil until it comes out the breather at the back  of the engine, and there is no oil returning to the tank.

I have drained the crank case three times and refilled the oil tank with fresh oil, but the result is always the same.
 
I had left it standing for rwo weeks with the oil tank filled with no sign of wet sumping.

All the external oil lines have been blown through with compressed air to check for obstructions and the pump filled with oil to prime it

When the engine is running I can see the oil level in the tank drop and when the engine stops the level remains the same, so the pump must be pumping.

Would I be right in thinking that the problem could be with 1. the oil pump, 2. the gasket behind the oil pump, or 3. a blockage in the pickup pipe ?

Any other suggestions would be appreciated

[Triton Thrasher]

Does it have a captive ball in the bottom of the crankcase scavenge pickup pipe? Is it stuck?

Is there a magnetic drain plug close to the ball, holding it down?

[CheeserBeezer]

I suggest the return side of the pump needs priming. Remove the flexible oil pipe off the steel pipe and pour very light oil (sewing machine oil, fork oil) into the pipe which goes down to the oil pump. Take the plugs out and rotate the engine backwards, very slowly. You should see the oil getting sucked down the pipe on its way to the return side of the pump. Keep doing this until you've managed to get at least half a cup full of oil down the return pipe. Now start turning the engine forwards and oil should start coming up out of the return pipe. Refit the oil pipe and restart the bike, fingers crossed!

[A7restore]

I will have to check the pickup pipe, but the drain plug is magnetic I will see if fitting a none magnetic one to see if it sorts out the problem

[Swarfcut]

   The oil pick up pipe in the sump has a non return ball valve, check that this is not stuck, clogged with carbonised oil and that the ball can rise against its stop. Turning the engine backwards will indeed draw oil back into the return side of the system, but unless this valve is held open, its a dead end and you may not suck enough down to prime the pump. Alternative is to feed oil up the pick up pipe with a pressure oil can and a length of tube. Stuck ball is a common fault on engines left standing, easy fix.

  You are correct in diagnosing possible reasons for non return. One other often overlooked is an air leak where the pick up pipe joins the crankcase. Plenty on the forum about the vagaries of the system, tales of sleepless nights, threats od destruction, you name it, it's all there.

 Swarfy.