Want to quit smoking

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Jun 14, 2007
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My 73 850 Commando has a smoking problem. I start it up and gas out half the neighborhood with blue smoke. (Runs very well though, and has 170# compression in one cylinder and 155# in the other, which doesn't seem terrible).

My half-a## guess is this means either valve guides or oil rings need replacing.

My equally half-a%% remembrance of things past is a quick and dirty check as to which is to run a dry compression test, and then a wet one (teaspoon of oil in the plug hole before testing) and if there's a significant increase in compression, it's probably rings as opposed to valve guides.

Am I on track?

Thanks folks - BrianK
PS, before someone suggests it, I don't have access to a leakdown tester. Thus the half-@## approach. Thanks - BrianK
Does it smoke all the time or just at startup?

If only at startup, could simply be wetsumping. Drain the sump before starting and/or ride it more frequently, and your problem is solved!

If all the time, then have someone follow you and note when it smokes. On acceleration = bad rings. On overrun = bad valve guides.

BTW, the difference between cylinders in your compression readings does not sound good even though the numbers are fairly high. I'd look into that although I doubt it has anything to do with your smoking.

good luck,
to smoke or not to smoke....

My 850 smokes likes there is no tomorrow. (I purchased it after it was standing in Wisconsin for a number of years). So I figured it was put away for that same problem. Did a head job with new valves and new guides, and for good measure I also installed new rings in the pistons which looked OK. It was smoking even heavier then before!!!! Took everything apart again and couldn't really see anything out of the ordinairy until a local chap with major Norton experience brought to my attention that if you look real good at the cylinder walls you see signs of oxidation imprints. This is probably caused by "sitting still" for many years and the mouse nest I found in the intake filter might also had access via an open inlet to the same cylinder and mouse pee is apparently also rather corrosive. So I'm in the progress of boring the barrel out to .20 over and install new pistons. According to the very knowledgable Norton chap (he is from Motoparts here in Alberta), this should solve my smoking problem(without going on the "patch"). Your smoking problem might or might not be sought in the same area. If you have no play in you valve guides and the valves are properly grinded in (no leakage) you might want to check the cylinderwalls very carefully and ensure that all piston rings have a sufficient "spring" to it (My new oil scraper rings were a lot flatter in their"spring" then the rest of the piston rings......) a new set!! Go figure! Fun happens...

Josh, Calgary, Alberta.
I suspect your intake valve seal is going up & down with the valve!
This is an easy 3- beer fix on a rainy saturday afternoon.
Remove tank, carbs & spark plug. Set engine so piston is comming up on compression stroke, Fill cylinder with clean clothes line (about 1/4" dia.) maybe 8 feet of it , Bring piston up by turning rear wheel in gear untill piston has pressed clothes line up against closed intake valve. Just enough line to keep valve closed near TDC. block intake & head passages with clean paper towel so you will not loose a keeper into the engine. Compress valve spring. Remove keepers & collet. Remove valve springs, (You need to remove the rocker arm & spindle first). Change out valve seal, reassemble.
Have a couple of different type valve seals on hand because some fit better than others on the valve guide. I like the ones that have a metal snap ring that holds the seal onto the guide but they need to fit into the inner spring diameter with out fouling when the spring is compressed.
Watch out when reassembling not to compress the valve springs too much and cut the new seal.
I bought my 1975 Commando from Motoparts last year and they said it was in fantastic shape.

It wasn't. Lots of sketchy fixes and half-assed repairs.

I would check into another place as well before committing to Motoparts for the repair as I had some grief dealing with them.
Coco said:
Lots of sketchy fixes and half-assed repairs.


I think they're all like that, no matter who you buy them from! (Except the CNW bikes). My ebay 750 had certainly suffered a lot of DPO abuse, although it looked all nice and shiny and pretty. I looked at a few overpriced local bikes before buying that one, and they were even worse :shock:

All part of the experience I guess!

Coco, Hi there. I remember your bike at Motoparts. From what I recall seeing, it was a very stock, original everything including the owner, bike. It wasn't perfect, but for unrestored it was sweeet!
Hope you've got it sorted


All right, Debby, I'll bite: What does DPO stand for? (D###head previous owner?) - BrianK

Hey, BTW, I just found your Debby and the Norton blog - (I assume there are not TWO mechanically gifted Norton loving Debbys in the Colorado area...?) Very cool (AND INSPRIRING)!
Hi Brian,

There are all kinds of D words that work for that, and none are flattering! The most polite version I've heard is Dreaded Previous Owner. I have some favorites that are less complementary! :lol:

Yep, the blog, http://debbysnorton.blogspot.com, is indeed mine. I suppose I should extend it to include the 850 project. I'll have to rename it to Debby and The Nortons I suppose :)

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