Smoking on acceleration

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Apr 15, 2004
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I took the bike on its first club ride today and it did great! The chassis feels solid, the front brake is really nice, and the bike feels very good. The only things holding me back were brand new tires and some sand in the corners.

But. The right cylinder smokes. The people following me said is was fairly heavy blue smoke whenever I accelerated. Nothing on overrun. I've known about the smoking for a while but didn't know the details. PO said it started when he had the top end rebuilt although he described it as "an occasional white puff of smoke" (he basically misrepresented everything). So I think it's safe to say they did something wrong when they put it back together, or botched the rebuild in some manner. The guy had a real aversion to replacing gaskets and oil seals so it could be something as simple as a valve guide seal that was reused or simply eliminated. The left cylinder is fine, no smoking at all.

Any thoughts on what it might be and how serious it is? I'd like to run it as is this season then tear it down next winter because the rebuild will probably take me several months. The bike starts easily, idles perfectly, and runs well otherwise. Compression is good - I measured 135 on one side and 140 on the other last year (don't remember which was which). Those reading were taken warm/dry.


A little blue smoke adds character!

My guess is that the oil control ring wasn’t installed correctly on the piston of the smoking cylinder. The oil control ring actually comprises three pieces and can be fiddly to install correctly. I doubt that the valve guide seal would cause much smoke on acceleration. Worn valve guide seals tend to cause smoke during high manifold vacuum situations, like trailing throttle.

I would keep an eye on the spark plug on the smoking cylinder to make sure it doesn’t oil foul. And it would be smart to carry a spare plug and wrench, just in case.

Personally, I would ride it as is and make repairs whenever it’s convenient.

I had this exact issue and it just needed new rings, did them 20 years ago and it has been fine ever since. A norton will get through 2 or 3 ring changes per rebore on the later ones with the good air and oil filtration. Sounds like the PO only did the cylinder head and did not do the rings on his rebulild, why it is always the right hand cylinder that seems to wear out more than the left must be due to the oil pressure, my guess is that on startup the left side gets the oil first and the right does not get it until the left is supplied and the back pressure increases to give the right side the oil.

An iffy oil seal on the inlet valve gives blue smoke on the over run as the cylinder suction is at its greatest due to the air route through the carb being restricted, it also shows as a puff of blue smoke as the throttle is reopened but it then dissappears, constant blue smoke under acceleration is normally rings.

Just ride it for the summer, in the UK it would stop you from being caught on the speed camera's :lol:
right side smoking

Conversations within the local club - the left side is usually the first to start smoking, it's the farthest from the oil pump. Oil is feed through the crankshaft from the right side (timing side). Kommando, you're in England so it makes sense it would be reversed from USA Commandos...driving on the left side of the road and all (humor).


Just ride it through the season then look at the rings when it's snowing.
Mine is starting on the right side again too....and this must be the 4th time I can remember....maybe using the sidestand gives the left side more lubrication...and why others have had the problem only on the left side...go figure. Not that I can see lubrication having much to do with it, though, most likely, the oil rings get "tired" from heat and can't scrape the bore clean anymore. I've never had to do a rebore, just hone and new rings. There has never been a "lip" on the bore top, so just the rings themselves were finished. This time maybe I'll have to do a rebore, ala Murphy......have to wait and see.

Anyway...the other fellows have it most likely correct...rings are bad.
I know the oil pump is nearest the right side but my theory is that the oil is not at full pressure untill it meets resistance and back pressure builds up from the left back to the right so the right side is for a few seconds oil free more than the left. Or maybe its because I do drive on the wrong side of the road and the camber has something to do with it :lol: .
it could be a valve oil seal, my friends 750 smoked like a pig on the left cylinder after a rebuild, turned out the left inlet valve seal was not down tight on the valve guide and went up and down with the valve. Worked like a little oil pump pumping oil down the valve.
As Tress was so nice as to once again use this forum to sell pills and drugs, this thread has once again caught my eye.
The problem of smoking on acceleration, which I had, and pulled my motor apart to cure...turned out to be a valve guide seal that was riding the valve stem....not rings or such...but my problem happened suddenly and Debbies' has been going on for a while...I'm sure she will let us know what it was.
some assembly required

We pulled the motor this weekend and tore it down. All the way down, including separating the crank :shock: The head is sitting on my workbench as we speak, awaiting disassembly. So I don't know about the valve guide seal yet. Found plenty of other problems though. Will post separately about them...

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