Right cylinder smoke

1974 850 Commando with 33 miles on it after completion of overhaul/rebuild.
I have blue smoke emitting from the right cylinder.

The cylinders were properly honed with new rings installed and were seated in the first hour of startup with rpm's continually being varied, up and down and up and down between 1500 up to 3500 but never allowed to just sit there at idle. Oil channels were primed before startup and lubricated where called for.

New valve seals were installed. New head gasket. The crankcase is "vented" including Mikes PCV breather and working. The valve seating was checked visually and looked correct, no hot spots or burn marks.
The oil level is just above the low marking on the tank cap indicator stick and the crankcase drained before startup.

I noted that the right side of the interior of the cylinder head was very black compared to the left side upon tear down and there was a tear in the right side of the original head gasket. I attributed this black/extra rich coloration to that head gasket damage.

The engine does not start smoking until it comes up to operating temp. The engine runs nicely but the smoke is obvious and there is oil in the right combustion chamber only.

I speculate (and hope) a faulty seal although I have little confidence this is accurate.
I really think the valve guide is bad or there is a crack in the head, although I never observed it when inspected after it is was off the engine.

This truly sucks considering all the time and attention that went into the rebuild. I should have done a complete overhaul on the head the first time I had it off.

My only other thought is to block off the incoming oil feed to the valve train on that side but can't see what difference that would make if oil is passing through a valve guide or faulty seal.

If you have any other ideas I sure would like to hear them. I was really hoping I got it right the first time......

slinger,
Check to make sure your valve seal on the RH intake side didn't pop off. Another check is to make sure the rocker spindles are in properly on the suspect side. Flat facing the valve. Check for oil passage through the cylinder from the head to make sure it flows down to crankcase. ie no goop blocking the passages.
Good Hunting.
Cheers,
Thomas

Same advice. Likely the RH valve seal.

You can block off the oil feed to the head to see if the smoke stops
This will tell you if it's a bore/rings problem if the smoke continues
Or a loose valve guide/ cracked head/valve guide seal etc etc if it stops

So I have exactly the same problem and have had for some considerable time. Bike runs fine I just get through a lot of oil. The usual culprits everyone mentions are valve guide seals, valve guides, piston rings. I have been through looking at these and I think there is another potential culprit and that is the oil return down the barrel on the right barrel.
I plan on doing the thin copper wire mod in the next month or two to see if that fixes it. There is a fairly recent thread with photos on this. If it does not fix it then I am not sure what I will do other than cross that bridge when I come to it.

baz said:

You can block off the oil feed to the head to see if the smoke stops
This will tell you if it's a bore/rings problem if the smoke continues
Or a loose valve guide/ cracked head/valve guide seal etc etc if it stops

Click to expand...

What steps/technique do you use to block off the oil feed into the head and stop oil flow through the incoming line...
Your attention is always appreciated fellas.

Lineslinger said:

What steps/technique do you use to block off the oil feed into the head and stop oil flow through the incoming line...
Your attention is always appreciated fellas.

Click to expand...

It was a very long time ago
But I think I found a spacer in my workshop that was a similar size to the banjo pipe/oil feed coming off the timing cover to the head
You could probably use a nut or some washers
Don't run it for too long without the oil feed though
On my norton it made no difference to the smoke the barrel had just been rebored and came with new pistons etc
People kept telling me the smoke would die down but it didn't hence the oil feed test
In the end it turned out to be the tag missing off the end of the oil control expander ring

Anything can block off the oil feed line to the head for this temporary test. A piece of shop towel screwed up and jammed into the output hole will work. It will only be there until that point in time where the engine started into the smoking. Yes if the smoking continues on and on then you have to pull the head for head gasket or piston rings inspection to locate the cause. Some won't do the test but I believe It's worth the risk.

I would try and poke the oil drain hole with a guitar string. I would also roll the piston down and look in the spark plug hole at the bore. Maybe you lost a piston circlip and it scored the cylinder... I've had that happen after a fresh rebuild...

Could be leaking past the guide outer bore and being burnt with the exhaust output. blocking the supply will show, or look into the port with the pipe off. Is that side plug more oily than the other? ,if so look for inlet seal or rings.

From the description, it sounds as if a lot of oil was entering the right cylinder before the disassembly and is still doing that now, right? In that case, it seems unlikely to me to be ring/cyl wall related since it smoked with both the old and restored cylinder bore/new rings. So that leaves the head...As noted in an earlier post, my first suspect would be the spindles oriented incorrectly. FWIW, my Commando came from a very prominent BrittBike purveyor who stated everything was rebuilt and in perfect shape - nearly NOTHING was right on the bike, including the spindles which were oriented incorrectly, causing just what you are experiencing...

If the spindles are oriented incorrectly, the valve box will fill with oil because it can't get out as fast as it is being pumped in. Also, as noted, a restricted drain can cause the same thing. If either of these things occur, the valve guide/seal will be submerged in oil and no seal/guide can keep the oil out of the combustion chamber under those conditions.

Really interesting and diverse feedback.
You gents have offered up more options for me to research and give a hard look before having to pull the head....or at the least come up with some accuracy to my troubleshooting directions.
Thank you for taking the time to respond and share your knowledge, much appreciated fellas.

I also have smoke from rt exhaust only, none from left, but only for a minute or so as the engine is warming up.
The possible causes listed in this thread seem like they would
not only cause smoke during first minute of warmup but all while
engine is running, cold or hot. Ive had wet sumping cause this
but if thats the cause why only smoke from rt cylinder??
Maybe i have wet sumping combined with a minor rt side ring or vlv seal leak? Would really appreciate any comments. Its getting
worse and a lot of smoke today! Also, its a newly rebuilt but broken in 71 commando 750 engine.
Thanks much, Tom

If you think there may be too much oil in the crankcase, check how much oil is in the crankcase.
See how much drains out.

Valve seals cause smoke typically at high vacuum levels, like when decelerating with zero throttle or at idle. Intake vacuum is sucking the oil past the intake valve seals. Rings typically cause smoke under load.

As noted in the thread the RH valve seal popped off. Smoke free after repair.

Not to be critical, but .... I participate lon 4 forums this one plus 2 Duc forums and the Griso Ghetto .... it seems when a post asks for help the majority of fix responses seem to go with worst possible scenario ... hope that does not happen here .... glad your smoke issue was an easy fix .... many years ago I had much smoke from both pipes ,was told needed engine work ....I lowered oil level in tank, cleaned oil from inside headers and pipes and been smoke free since .... quite often a fix while elusive is not that difficult or complex ....

Wish mine had been a simple fix (I don’t actually know what it was).
I lived with a smoking right cylinder for about 2 years, played around with all the usual suspects, from oil tank level to rocker spindles to Indian rope trick with intake seals. Last winter took head off, replaced piston rings, honed cylinder by hand with coarse sand paper, new valves and guides new guide seals. Opened up drain line a little, modified valve spring cup. Now it doesn’t burn oil at all, or at least very little.

Mine was on the left side and new rings and a hone cured that, the blue smoke came out during acceleration which was the clue to it being the bore.

kommando said:

Mine was on the left side and new rings and a hone cured that, the blue smoke came out during acceleration which was the clue to it being the bore.

Click to expand...

Informative responses but not addressing my specific question.
My smoke is only on rt side and only for a minute or so after cold start. I suspect wet sumping and that would explain most of it
but why only the right side?? Im asking for thots so i know what all to inspect and what not to bother with. When its smoking it
does it at idle and proportionally more as you rev it but only for
a minute after a cold start. All other times, no smoke.
Thanks for any further comments, Tom