Left side smoking

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Jun 12, 2007
My 74 850 commando smokes when it's hot. Not quite sure what this is. I've tested the compression and it's all good.

Is it likely to be the piston rings or valve guides?

Any suggestions/experience would be very helpful.

Post results of your compression tests, along with methodology used.

The info in your first post isn't likely to get you a proper response.

One guess - your crankcase breather outlet might be closer to the left carb intake than the right one, and it's sucking up excess oil mist into the left cylinder.
To be hones with you, I tested the compression quite a while back and can't quite remember the results just that there didn't seem to be a lack of it on either side. Thanks.
A proper compression test, followed by pouring a spoonful of oil into one cylinder and re-testing, then the other cylinder, etc., would show if there was a relative increase in compression with the added oil, indicating weak/worn rings.

No relative increase in compression with added oil points to the valves/guides.
As Paul says need more info. Questions like this can lead to answers like this off another forum vary scary answers in deed. Thanks to Dave C and all the work on his site. This would be your worst case nightmare come to rest in your world. Pulling the head would revel if your in the 50% bracket.

Quoted from BB.com
Dave Comeau

March 27, 2008 07:35 AM Dave Comeau
You might know that probably 50% of 850 head are cracked by the intake guides.


A lot of norton heads are new with sloppy guide clearances.

Unless you have had this head inspected and if needed by some one that "knows" norton, than if this was my bike I would alway have in the back of my mind that the head is the culprit.
I have over half dozen 850 heads on my shelf and not one is fit for use without rework. some are very low mileage also...
With a bit of luck you may find all you've got a lose valve guide seal.
With the left hand inlet valve closed using a small mirror and light, look between the spring coils, you should be able to see the chrome valve stem and the top of the seal. If you can only see the seal then its come adrift. Check the other side and you will see a difference
The seal can be replaced without removing the head. Do not reuse the seal and the replacement should be of viton.

Fingers crossed.

That figure of 50% cracked sounds very high. Did this occur initially or can it happen at any time ? My RH4 head has given no trouble but it's had quite a hard life.

I have to say that I've never spoken to anyone who has mentioned this problem. Maybe Norton Villiers were just very careful to ship the poor castings as far away as possible ? :oops:

"Mr. Poore, another cracked one" !

"Send it in Berliner's next consignment then" ! :evil:
Scary Stuff ! My 750 is bored for 850 style guides as well.

Perhaps I've just been lucky although having had the work done by Norman White is probably a factor.
I'm very curious to see what a possible solution to this problem is.
I have the same problem (just with the right cylinder). Got a head job done (new guides and valves) >> still smoking.

Oversized,(20 tau), the cylinders and got new pistons/ piston rings and............ Yes, still smoking!
was advised to ride , ride and ride it some more to "settle the rings" .

I only did 100 miles now but it still smokes like a Cuban sigar!
I suppose I keep riding it. Maybe I'm naive, but deep down I think it will still keep smoking (embarrasing at traffic lights....) I examined the head rather carefully with a magnifying glass but could not find anything that looks like a crack. What the heck???
Have you tried running the motor for a short time with the rocker feed blanked? If the smoking stops its the guides or guide seals. If not it's the bores etc.

More info?

So I've had to take the head off anyway. Can you tell much from the inside? There's a great deal of carbon on the exhaust valve as expected. just curious if there's any obvious test I can do with the head off.

Look for oil on the cylinder walls, on the inlet valves and stems. Carbon stuck on the inlet valves (port side). And any cleaner or darker areas on the casting.
A penetrate dye test for cracks radiating out from the intake valve guides. A liquid test to see if oil is finding it's way around the Valve guide at the fit into the head. If you read through the links provided you would find much to do.
Reading up on the difference between oil burning carbon and gas fouling carbon before you remove the carbon comes to mind. Unless you would like to all this again real soon. You need to do careful detective work.
Watch your piston domes ! Are they clean without any carbon close to the cylinder walls, then your piston rings are worn( they pumped up oil fromthe rings cleans the piston dome ). If you have a lot of carbon on your pistons, the oil came from the top.
Is it blue smoke or black ?

Greetings Marco
Whitish smoke. It'sdefinitely oil, there's carbon all over the place. I'll take some pictures and post them.

Hard to say. I would think since the rim of the pistons looks wet, it would be from oil collection on the cylinder walls from the oil scrapers not doing their job, or the carbon not allowing them to do so.

But then again, valves could be bypassing oil and not burning that collecting oil around the rim of the piston. I gotta believe most of the flame heat is condensed in the hemispherical portion of the head, not the corners of the cylinders.

Maybe a picture of the cylinder walls?
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