If you are using the fuel pump hole on the back of the block (which by your description of rear engine breather I'm assuming you are) as a vent, then you need to block it off or get one of our host's breather adaptors to prevent wet oil from being thrown out of the engine.

It's not a breather...

Thanks all.....

put some Marvel Mystery Oil in you gas and oil. It loosens the carbon and lets the rings expand. It won't hurt and may help.

Sounds like you need to run a hose to a vent tank to catch the mist and let it settle out. Lots of them on ebay and your favorite speed shop. Just put it where you can empty it.

The filter is to keep dirt from getting sucked in as the engine cools down. You could hook up a positive crankcase ventilation (PCV) valve onto the breather pipe which sucks the fumes back into the inateke manifold. BUT the oil mist coming out is from piston blow-by which maybe indicates its time for new rings. You might be able to alleviate the problem by adding an extra breather, either on the timing chain cover or on the clutch housing. A Mini motor can never have too many breathers.

Question about the rear engine vent. I've got the small K&N filter but still seem to get oil blowong out over the firewall.  Any suggestions to stop this?