Commando 750 oil loss, part 2.

A few weeks ago I posted a summary of a total oil loss from my Commando, (Tragic Oil Loss), whilst returning home from a ride out. Oil everywhere underneath the bike, with the primary chain case full to the brim with oil. Obvious conclusion: crank-case oil seal failure. Of course it wasn't, the golden rule being whatever you think it is, it is always something else. In this case one of the three Allen bolts that secure the inner chain case to the crankcase had worked loose, and fallen out. I blame myself for this, in that I have fitted the Alton starter kit , and in securing the chain-case I think I was too hesitant about tightening the bolts too much, being fearful of stripping the threads. But the hole is tiny, just a few mill, and it seems strange that so much oil would just weep out over a few miles of travel. Anyway, having reassembled the starter and primary drive, I fired her up, thinking that was the problem solved. No it wasn't. This is the situation as of today, (April 28th). With the engine running, no oil flows back to the tank via the oil filter return line. I have changed the filter, thinking it might be blocked, but on testing further there is no oil coming through the feed pipe to the filter from the engine. What is happening is that the oil gets back to the tank via the crankcase breather. Presumably the pressure builds up as the oil accumulates in the bottom of the crank case, and having no exit point, after about 20 seconds of the engine running oil is forced up the breather pipe and so returns to the oil tank. (Just as a note relating to the original problem, being under such pressure, I am guessing the oil squirted through the missing bolt hole, thereby filling the chain case). Before I strip out the pump, has any one any idea why the scavenge is failing to scavenge? I should add that my oil pressure gauge shows bags of pressure, (70psi on start-up, running at 40psi when the oil is warm), so it is not the pressure side of the pump that is at fault.

I am scratching my head, which is not unusual, even after 25 years of running the old girl.

Andy Bone