Wow...that's a LOT of smoke. Frankly, if it was me I'd just bite the bullet and pull the head/barrels and basically "start over." I originally said valve seals but that's an awful lot of smoke for seals unless they are totally missing AND the valve guide/valve stem clearance is really excessive. It could be a poor cylinder wall/ring interface due to improper finish on the cylinder causing the rings to have never seated properly.
To be honest, if I saw an engine doing that, my first reaction would be to just consider it clapped out and needing a rebuild. Was anything other than the head redone? If so, is it possible that the wrong size rings were used? IOW, if perhaps the cylinder was bored at some time in the past to .010, .020 or whatever oversize but the rings were not of the corresponding oversize? Or, as I said, maybe the cylinder wall was not properly finished for the new rings. The proper bore finish is related to the composition of the rings. IOW, cast iron rings require a different bore finish than moly rings. OR, is it possible that new rings were installed with no bore refinishing? The same thing will happen - poor oil control.
The part that is most interesting to me is that you stated that the leak down and the comp test show good results which would then indicate that cylinder sealing, which includes the head gasket, valves (to valve seats) and the piston rings, is good. That's why my initial reaction was valve seals/guides since that is not "checked" by those tests.
In any case, UNLESS you check and find the valve seals missing(!) I'd just tear it down and rebuild it - checking to verify the cylinder diameter and ensuring the correct size rings are purchased and ensuring the bores have a suitable finish for the new rings. Off the top of my head, I can't see how cracks/porosity in the head can cause the problem because that would show up in the two tests unless the cracks/porosity are in the valve box which would allow oil into the combustion chamber when the valve opens but the tests would not show that, just as they would not show bad valve seals/guides.
Good Luck!
Hi, I was hoping you did not do the copper wire around pushrod tunnels , with pliobond , so ...but you did ! I have a 920 built by a mix of Pete Lovell and Les (who did what , ???), and yes it was smoking , but just a bit a the start, and far less than yours, after putting this engine off the bike for years , I had it apart last summer , checked everything , and do the thin wire with new head gasket and gasgacinch (no pliobond available in France!!), and that time tho smoking a tiny bit , everything was ok , I know those 920 barrels had this reputation, so I think I am ok now .....sorry if I do not help you so more but I think you have to lift the barrel again and check rings , and clearances !
Head checked, rings good, compression test done then there is one area I would look at next - a barrel shaped bore. New rings could have masked this issue for a while. Compression test can't detect it. Usually incorrectly honed barrel causes this, some are so bad that the first pass on re-bore will miss the centre section.
I verify that just wet sump splash can over oil bore even good oil ring can't wipe so smokes like that till flushed out. Suppose a broken oil ring could leave enough to smoke on one side only. Leak down tests are not complete w/o checking somewhat down on either side of TDC which is serious task in Commando jug under pressure, soon as .005* off TDC. Its entirely possible some oil return issue only showing up on the less well sealed side but not likely. As stated a few times prior, in for a penny in again all the way.
How's the sump magnet look? Most of it is flake/splinters/nano dust off rings bores and bearings rollers/races. Bore scratches or wear enough to smoke would leave extra. Let us know once in hands again : )
I recall reading that if a rocker arm shaft is put in with the bleeder hole facing the wrong way it pumps way too much oil into the top. You might check that.
I also thought that maybe the drain hole for the head is clogged at the base of the barrels where it enters the crankcase causing oil to back up as it's fed into the head. Maybe a blob of silicone went down the hole and is stuck.. I wonder if there's a way to poke that hole with the barrels in place to make sure the hole is clear.
That test where you disconnect the oil feed to the head may tell you if the leak is coming from the head or the piston side, but I wouldn't do it without consulting someone knowledgeable...
And lastly, Pulling the barrels isn't really that big of a deal.
Could be the mufflers are loaded up with oil now so you'd be unable to tell if it gets fixed. You really need to ride it for some distance. Revving it in the garage is not so good for the bike either. Do you have a big floor fan?