71 commando likes to keep oil at "low"...


Jul 31, 2006
Hi, I have finally gotten my 71 commando on the road, and i notice that if i have the oil anywhere except 'low' on the dipstick, it will spit oil out of a breather until it returns to low.... is this just the way my bike is going to be or is it indicative that i may have some plumbing messed up?

My -73 Interstate (90000 km) had the same behaviour.
Going really slow, 50-55 mph was the only way to keep the oil level.
Anything above 1/3 throttle,or pulling past 5000rpm, made mine eat oil.
The oil was blown to the oiltank by the crankcasepressure,and into the aircleaner . The engine chew it up,and mostpart was thrown out with the exaust. I finally cured this by leaving my worn shortblock to a specialist. He rebored the cylinder and fitted new pistons,made a regrind on the crank, and finally, put it all together with new mains.
The oil-eating was gone, and the engine is more "oiltight" in other aspects to.Before there was "some" seepage here and there!
You might run the engine with the oilcap removed to check crankhouse pressure.One other thing to try, is a regular compression test!....if you dare!

Benton said:
You might run the engine with the oilcap removed to check crankhouse pressure.One other thing to try, is a regular compression test!....if you dare!

I get a lot of oil pressure back into the oil tank. it sort of makes a mess and showers everywhere if you have the cap off if the idle is high. should I be able to ride around without a cap without it spilling all over? I guess I could try plumbing the oil radiator in a different place to give the oil a more gentle return back to the oil tank? for a little while i thought that having the breather dribble onto the chain would be like a lazy-man's chain lube, but then i realized it was also getting oil on the rear brake and tire, hahaha.

also, given how hard it is to kick start this bike, i don't think i am ready to do anything that brings up the compression yet!!!!!
Same behavior with my 1971 Fastback....Every Norton person I have discussed this with to date says the "breather" in fact is a "spitter"...general consensus also seems to be the "L" marking is the beginning of the "normal" oil level, the mid point between "L" and "H" is the highpoint, and over the mid point is over filled, and will increase the spitting....Speaking about the breather, when I got the bike back after being restored it had a 1/2" breather line running from the front left hand side of the crankcase to the oil tank. Bike was leaking oil from the main seal, and since I noticed that this was the case only when the bike had been run on the freeway, but not in stop and go city traffic, I suspected this was a breathing issue. I inserted a PVC value in line on the breather pipe, and this reduced the oil leak but did not stop it. I then read on the Norton Club site about better results with a brake booster valve (motormite/Help power brake booster valve vs. a PVC valve) and installed one of these. This reduced the leak somewhat again, but I am somewhat concerned about the valve itself. It is made out of an ABS type plastic, and has a rubber flap which covers a circle of holes to stop back pressure. It reacts somewhat faster than a PVC valve, which may be the important thing since the back pressures seem very light (estimated to be about 2 psi in the article on the Norton Club site) but with the heat of the engine oil I am somewhat concerned about the longevity of the valve....
ah, thanks for the feedback, it is making me feel a lot less leery about my norton!!

I wonder why the breather tube isn't just routed to dribble back into the oil tank? i guess having such a long pipe like that would prevent the breather from restoring vacuum as quickly?? shrug!
You could always run the breather from the oil tank into a beer can or something for that real cafe racer look. (Don't think that I'd tip it back in again though).

On long runs, I fill my 850 Mk111 up to the full mark and it doesn't cause problems. If seriously overfilled after not allowing enough time for the sump to scavenge properly, it will pump out down the extended breather along the rear frame. After that, I suspect it soon finds a home over the windscreens of overtaken cars :)

Leaving the cap open gives a false impression because the oil coming out at the top of the return line meats the pulsing from the breather inlet to the oil tank and forms a crude venturi effect blowing mist out of the filler aperture.

As an aside, have you ever wondered why so many Dominators have a strip of rag wrapped around the filler cap ?