Strange problem, blowing oil

Finally admitting defeat on this one and hoping some of you guys can throw a fresh light on an issue.
Scenario is, a friends 750 Commando which he built from big lumps (a non runner when he bought it), its a late 73 non Combat, breather on the back of the crankcases,
First issue was after he rebuilt it no oil return, this was traced to silicon in the oil gallery, this was cleared and a brand new oil punp fitted at the same time (all galleries are now clear).
Oil returns fine now but after 2 minutes of running oil appears to come from the front of the head. This is like dollops of clean engine oil
I've stripped and built it now 4 times myself checking everything I can think of but still the same issue. Followers are in the right way, pistons are a good fit, new rings beeen gapped properly, bores honed and all surfaces checked for true by a respected engineer.
There appears to be a build up of crankcase pressure immediatley and within 2 minutes the bike smokes badly, when you strip it the bores and exhaust ports are full of clean oil (not burnt combustion oil). No obvious cracks but some evidence of casting blow holes (pin pricks) on the head face.
Any ideas greatfully appreciated

Hi there
Rocker shafts the wrong way round, breather blocked ?
Piston rings upside down ? crack in head ? studs in front of head loose/bottomed out in head?
pin hole in pushrod tunnell?

Kev

Gino, I had nearly identical issues immediately after my rebuild. Even down to the silicon in my oil pump. Your post references two problems, they may or may not be related. Even after decking the head, I would get oil weeping from the front of the head near where the push rods go thru the barrel. It is a common problem from reading over this forum, many nortons share this flaw. I went thru 3 head gaskets before I stopped, or at least slowed this problem down. I settled on a crushable copper gasket, which I sprayed lightly with metallic paint, then sprayed with gasket tack and immediately reassembled. This leak may have nothing to do with the other problem. Or it might.
If they are related, then it is a top end oil issue. Oil may not be returning from the head properly, and the oil then builds up and is forced into the cylinder and also out that weak spot on the head gasket. Recommendation: remove valve inspection cover and start bike, look to see what is going on in there. If oil not returning, you will know, it will be on your floor. If inconclusive, here is sure fire test: completely disconnect the oil fed line to the head and start bike. Bike can run long time without oil going to head, no damage will be done. If runs for 5 min and no smoke and no leak, you have definitely narrowed down to head issue, be it guides or plugged drain.
To be continued

Sorry, posting from my phone, needed to start new reply, screen filled.
continued....
If you rule out the top end being your problem, then most likely bore and or ring issues. My bike went thru major smoke issues after rebuild, and I swore the rings and jug could not be the issue, everything was replaced and machined. However, I was proved wrong and the machine shop did not do a good job the first go around. I had to go .040 in the end with new pistons and rings, problem went away. If you are getting oil past the rings, you should also have a problem with compression leaking into cases, have you seen oil being forced out the breather? They typically go hand in hand. Have you done a compression check on cylinders? If low, points to rings/bore. That reminds me, are you experiencing oil in both cylinders or just one?
In summary, check head first. Easy and most likely the culprit. Second, don't trust other peoples work and sometimes even your own. when I went thru this same issue I would have bet money it could not be pistons and rings, they were new and jug was machined. I was wrong, and had to have it punched again.

Thanks guys, both cylinders are the same, I'll go through your list (some of which I've checked) and keep you all posted, won't be this week though, pig sick of the sight of the thing at the moment, got loads of other stuff to do on my own bikes as well!

Another possibility is the head gasket could be installed upside down. The small oil drain hole would then be blocked. Definately run the bke with the head oil feed disabled as a test.

Gino Rondelli said:

Thanks guys, both cylinders are the same, I'll go through your list (some of which I've checked) and keep you all posted, won't be this week though, pig sick of the sight of the thing at the moment, got loads of other stuff to do on my own bikes as well!

Click to expand...

Geno,
what about removing the head, which is on the cards anyway, bolt a plate over the push rod holes on the head face, and block off the other holes..eg tappet covers etc ..then get some air pressure into it...them some soapy water applied to the outside surface's.this may show up a porous head...carnt see a man of your experiance not fitting a head gasket properly after 4 goe's :!: unless the head/ cylinder face's are warped.

My mate had a commando with no drain champhers on the cam followers, so another item to check :?:

Gino,
The clue might be clean oil in the exhaust ports. Perhaps if the exhaust guides are slack and clean oil is migrating down into the exhaust ports, when the throttle is closed it's getting sucked back into the combustion chamber. Remove the exhaust pipes and fire her up.

It'll be something daft it always is.

Best of luck and all the best for the New Year.

Cash

Gino Rondelli said:

There appears to be a build up of crankcase pressure immediatley and within 2 minutes the bike smokes badly, when you strip it the bores and exhaust ports are full of clean oil (not burnt combustion oil). No obvious cracks but some evidence of casting blow holes (pin pricks) on the head face.
Any ideas greatfully appreciated

Click to expand...

Taking your observations at face value, look to the breather. Is the breather choked off with silicone bits? Is there a breather valve installed (backwards)?

Are you working to a freshly honed or galze busted barrel surface. Maybe the rings are refusing to seat.

Oil in the cylinders (burnt or unburnt) could be from the pistons/rings or intake guides. Open the intake valve cover and see if oil is pooling in the valve pockets while running or do a simple drain back check with warm oil and a warm engine; pour it into the intake valve spring pockets and see if it drains and how fast it drains.

I had installed a new set of OMEGA piston (10.25:1) back in the 70's and it began to smoke and ping when hot on one of the cylinders. Upon inspection I found that one piston did not have the drain holes drilled for the oil control ring. I drilled them and problem solved.

Not sure what you mean by front of the head.....exhuast port side?

Thanks for all the suggestions guys, after a day spent checking various scenarios we think the oil is coming down between the guides and the head, head is now off for a complete strip and check. When its sorted I'll let you know what it was :roll:

If you don't have a one-way breather valve, install one. No matter what the final cause, it WILL help.

My Trixie Combat had intermittent smoking and oil on the side covers and a puddle of oil from the barrel base. Got the base re-sealed then the smoking out LH pipe got worse but cleared up nice on the hwy after a few minutes. Finally figured out I'd not re-troqued the head in couple 1000 miles and now see oil at head seam. So I suspect the smoke is oil leaking via the push rod tunnel area into combustion so will re nip head and see. Mainly posting to tell ya that Hylomar works good on both sides of the copper head gasket so might try that in your case while keeping ahead of the bolts slacking off form gasket crush down. Trixie stayed dry and smokeless all summer even on 500 mile Texas trip and back so upsetting to see smoke and oil this fall and winter.

hello, i've had this problem with my 850 mk2 for the last 20 years and thought it was something i would have to put up with. i've had all that work done, new guides, skimmed head (had the same small pin holes in the face) i've recently read on the internet that some one has used a none return valve in the breather pipe which has cured the problem, it relives the crankcase pressure, valve is off a car brake system. may be the answer as my bike got a lot worse since going to 920