Oil pumping through breather

I have a problem with oil pumping into the tank via the breather hose. I have recently rebuilt a 72 Roadster and she starts right up with no obvious issues. However, after a few minutes of idling oil starts pumping into the oil tank via the breather hose. I get the sense that oil is not returning to the tank via the pump. You probably know that the 72 has a breather on the back of the crankcase. I thought I might have over filled the engine so I drained the crankcase and oil tank and refilled with 6 pints but I still get oil pumping into the oil tank so overfilling appears to not be the issue. I have an spin-on oil filter conversion fitted but I don’t see how that could cause this issue. I checked the routing of the hoses and they appear to be connected corrected as per the workshop manual. Any thoughts on what to check? Pressure release valve? Oil pump not scavenging ? I’ve attached a picture with a screwdriver showing where the oil entering the tank. Thanks.

One cause could be the oil pump paper gasket fitted the wrong way round and covering up the oil pump scavenge pump port. This means that the only way that the engine sump oil gets back to the tank is by the breather. The engine rotational motion would be literally pumping the oil back to the tank through the breather.

One way to check this is to see if the scavenge pump is visibly pumping oil into the tank when the engine is running. The oil path is out of engine, to filter, out of filter to the inlet metal pipe in the oil tank.
In the photo I attached, the arrow points to the flattened pipe inside the tube. You should see oil coming out from just under the top of this flattened pipe.
Dennis

DennisMo said:

One cause could be the oil pump paper gasket fitted the wrong way round and covering up the oil pump scavenge pump port. This means that the only way that the engine sump oil gets back to the tank is by the breather. The engine rotational motion would be literally pumping the oil back to the tank through the breather.

One way to check this is to see if the scavenge pump is visibly pumping oil into the tank when the engine is running. The oil path is out of engine, to filter, out of filter to the inlet metal pipe in the oil tank.
In the photo I attached, the arrow points to the flattened pipe inside the tube. You should see oil coming out from just under the top of this flattened pipe.
Dennis

Click to expand...

you nailed it!!! I checked for oil flow and there was nothing so I removed the timing cover and oil pump. I had installed the oil pump gasket incorrectly and it was masking the hole. Thank you for your help.

DennisMo said:

One cause could be the oil pump paper gasket fitted the wrong way round and covering up the oil pump scavenge pump port. This means that the only way that the engine sump oil gets back to the tank is by the breather. The engine rotational motion would be literally pumping the oil back to the tank through the breather.

One way to check this is to see if the scavenge pump is visibly pumping oil into the tank when the engine is running. The oil path is out of engine, to filter, out of filter to the inlet metal pipe in the oil tank.
In the photo I attached, the arrow points to the flattened pipe inside the tube. You should see oil coming out from just under the top of this flattened pipe.
Dennis

Click to expand...

Nicely done..

Glad the problem is solved!!!

A thread like this illustrates why this Forum is outstanding. Everyone wants to help.
I consulted the forum on my recent engine rebuild which was my first air cooled.
And for that I am grateful. It saved making many mistakes.

No leaks and engine not seized after 900 miles.

I will soon consult the forum about optimizing my Mikuni tuning!
And then there is the rotational ticking from my front brake rotor that despite new pads twice, still happens.

Dennis

Last week I installed my oil pump on the 1972 being rebuilt. I am fairly certain the parts diagram I looked at at Andover Norton shows the gasket incorrectly oriented.