750 Commando LH cylinder smoking at idle (warm?)

S

StephenP

Joined
Oct 23, 2021
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Good morning, I have a 69 750 ‘S’ and there seems to be an issue with it smoking at idle from the left cylinder. I am hoping it’s not a ring or bore issue, it runs very strong and i have had people follow me and they say they do not see any smoke under load, and turning my head back i don't seem to notice under load.

However at a stop light (especially when facing up hill) my lower pipe (left cyl) seems to smoke some even when i rev it. Again, i don't believe it does it at cruise or acceleration

Traditional wisdom says this is indicative of a bad valve stem seal

The bike has around 10k miles, to my knowledge it’s completely original, and the engine has never been apart. Original other than the clutch, battery and tires. It was a one owner bike. I keep my oil between L and H on the stick (after warm up to ensure accuracy)

i can’t speak much to the actual consumption of oil vs mileage. I ride it around town sporadically and give it a good beating when i do, it’s not like I put hundreds of miles on it, but it never sits more than a week to three weeke. Have added a half quart here and there.

I really do not want to go any further into the motor than needed. It’s s great running strong bike and these things tend to snowball. In for a penny in for a pound and i will have a full build on my hands, and then why not upgrade everything, before you know it, the bike is blown apart for a year.

I have been a Norton owner since 2021 but honestly other than replacing the clutch and fooling with carbs, and replacing a kick starter pawl on my previous 850, i have not had to do any real work to the bikes. They have always ran for me.

My only concern is the side stand on my 69 lets it lean a lot more than seems right, i worry maybe fuel boil or fuel level in the bowl may be causing it to flood the cyl and wash the rings? I usually shut the gas off about 1/4 mile before my destination and let it idle a bit before killing the engine

Surely i am over thinking and it’s just stem seals?

I am an airplane mechanic by trade, with everything from radial to recip to jet/turbo prop experience, i have built several big and small block chevys, traditional Pontiac, and datsun z car engines, so mechanical aptitude is not in question, but i have never had to go inside a brit bike engine, let alone a norton, so i am trying to tread carefully.

Any thoughts?
 
Unless it’s really bad, I just wouldn’t worry about it Stephen.

I might allow my worry energy to extend to trying some different oil, but other than that I’d thrash the thing and just add ‘fix burning oil at tickover’ to the list of things to do when the engine really needs intrusive repair.
 
Definitely oil smoke?
I had a similar issue which turned out to be a sticky float needle. Grey smoke rather than blue in my case.
 
+1 on @B+Bogus

Make sure it's oil and not too rich smoke. In other words, if you're sure it's at less than 1/8 throttle, it's more likely the air screw in too far or wrong float height or sticking.

Do not run a Commando on the side-stand! The intake side oil drain is on the timing side so the intake area over-fills with oil on the drive side. Also if the float heights are not correct it is possible to cause over or under fueling although less common.

So, if it doesn't start on the 1st or 2nd kick and/or doesn't idle smoothly without blipping the throttle, when sitting upright, fix that before anything else.
 
Last edited:
StephenP said:
Good morning, I have a 69 750 ‘S’ and there seems to be an issue with it smoking at idle from the left cylinder. I am hoping it’s not a ring or bore issue, it runs very strong and i have had people follow me and they say they do not see any smoke under load, and turning my head back i don't seem to notice under load.

However at a stop light (especially when facing up hill) my lower pipe (left cyl) seems to smoke some even when i rev it. Again, i don't believe it does it at cruise or acceleration

Traditional wisdom says this is indicative of a bad valve stem seal

The bike has around 10k miles, to my knowledge it’s completely original, and the engine has never been apart. Original other than the clutch, battery and tires. It was a one owner bike. I keep my oil between L and H on the stick (after warm up to ensure accuracy)

i can’t speak much to the actual consumption of oil vs mileage. I ride it around town sporadically and give it a good beating when i do, it’s not like I put hundreds of miles on it, but it never sits more than a week to three weeke. Have added a half quart here and there.

I really do not want to go any further into the motor than needed. It’s s great running strong bike and these things tend to snowball. In for a penny in for a pound and i will have a full build on my hands, and then why not upgrade everything, before you know it, the bike is blown apart for a year.

I have been a Norton owner since 2021 but honestly other than replacing the clutch and fooling with carbs, and replacing a kick starter pawl on my previous 850, i have not had to do any real work to the bikes. They have always ran for me.

My only concern is the side stand on my 69 lets it lean a lot more than seems right, i worry maybe fuel boil or fuel level in the bowl may be causing it to flood the cyl and wash the rings? I usually shut the gas off about 1/4 mile before my destination and let it idle a bit before killing the engine

Surely i am over thinking and it’s just stem seals?

I am an airplane mechanic by trade, with everything from radial to recip to jet/turbo prop experience, i have built several big and small block chevys, traditional Pontiac, and datsun z car engines, so mechanical aptitude is not in question, but i have never had to go inside a brit bike engine, let alone a norton, so i am trying to tread carefully.

Any thoughts?
Click to expand...
have you done a compression test?
Post the results.
Add a teaspoon of oil after, re-test, post the results.

Also, IF you have someone you trust to ride your bike, and you follow it on another, OR, someone you know has observation skills and engine knowledge to follow you. Also, at night, with headlights behind you, smoke will show up in your mirror quite readily.
All to confirm/rule out smoke at cruise/accel.
 
Last edited:
It's an old British motorcycle. My first reaction is just ride it if it is not smoking at cruise and it idles smoothly. If it is loading up smoking at idle and stalling you know it is more than likely carburetion. If it just smokes a little coming to a stop after a couple of miles, the sump may have been overly full from sitting a couple of weeks. Are you draining the sump after the bike sits for 3 weeks? I'm not sure if that year had a sump drain plug. If it does you could check to see if a lot of oil drains out of the sump after it has been sitting for weeks. Time for a reed valve crankcase PRV update maybe, and or one type or another anti-sump update.

Anywho, pulling the head off for a refresh is a PITA first time, but not turbo prop science. A borescope might tell you something before pulling the head off. Embrace the update snowball, or leave it basically stock. In stock trim a Norton works pretty good.
 
If it is rings then left side typically goes first but the smoke appears on acceleration only. So likely just some oil pooling somewhere when the engine is idling, the top end can run without being fed oil for a short time so if it stops when you block off the top end oil feed then you know its head related.

I would wait until it got worse if its running well, then the fault will be more obvious or it may never get worse which is even better,
 
Hi Stephen,

My experience is British Classic cars and I have rebuilt them and their engines etc

I was new to Norton and air-cooled when I got my 850 in 2023.

I think I overheated mine on a very hot day in stop and go traffic for a couple of hours and from that time on
I got massive blowby with water in the oil and water blowing out of the oil tank vent at cold idle.
I could burn it off with a very long run but would come back with shortruns.

I recently pulled my head and cylinders and found massive cylinder wear of 15 plus thou.
If fact so worn the machine shop says to try go from its current plus 40 oversize to plus 60 might not work
So I bought a set of used cylinders and have them already bored to plus 20 with new pistons and rings.

Now I have the head at the machine shop and it needs new guides so I have new valves, springs and guides on their way.
The guides were very worn

It's my first time pulling an air-cooled apart so I have asked the forum lots of questions. Like sealant for the cylinders to crankcase flange.

In your case I agree with the previous suggestion to start with a warm engine compression test, then repeat with a little oil in the cylinders.

Then, if that points to the valve, there is a drain hole for oil return from head to sump that can be looked at.
I don't have my head handy but think it might be on the right side. Even though it is likely on the right side, it could contribute to the problem.
Wise Greg Marsh might comment on if this could be a factor.
I would try figure out a way to blow that passage out with the inlet valve cover off. I don't know if it can be blown out with the inlet valve cover off.
It could be that slow draining is a problem (not "the" problem) and that clearing the oil drain might reduce the burning.

I think Greg Marsh has told me the head oil drain empties into the timing cover so if visible there you after you take off timing cover, you could suck the drain clear (as opposed to blowing it.)

Some may suggest a little STP to thicken up the oil, but I don't think that will clear up a very visible leak. Maybe...

So if you do end up opening it up, I have just done all the learning you will do and can help.
There is lot of interesting stuff to learn and this forum is so helpful

Dennis
 
DennisMo said:
Hi Stephen,

My experience is British Classic cars and I have rebuilt them and their engines etc

I was new to Norton and air-cooled when I got my 850 in 2023.

I think I overheated mine on a very hot day in stop and go traffic for a couple of hours and from that time on
I got massive blowby with water in the oil and water blowing out of the oil tank vent at cold idle.
I could burn it off with a very long run but would come back with shortruns.

I recently pulled my head and cylinders and found massive cylinder wear of 15 plus thou.
If fact so worn the machine shop says to try go from its current plus 40 oversize to plus 60 might not work
So I bought a set of used cylinders and have them already bored to plus 20 with new pistons and rings.

Now I have the head at the machine shop and it needs new guides so I have new valves, springs and guides on their way.
The guides were very worn

It's my first time pulling an air-cooled apart so I have asked the forum lots of questions. Like sealant for the cylinders to crankcase flange.

In your case I agree with the previous suggestion to start with a warm engine compression test, then repeat with a little oil in the cylinders.

Then, if that points to the valve, there is a drain hole for oil return from head to sump that can be looked at.
I don't have my head handy but think it might be on the right side. Even though it is likely on the right side, it could contribute to the problem.
Wise Greg Marsh might comment on if this could be a factor.
I would try figure out a way to blow that passage out with the inlet valve cover off. I don't know if it can be blown out with the inlet valve cover off.
It could be that slow draining is a problem (not "the" problem) and that clearing the oil drain might reduce the burning.

I think Greg Marsh has told me the head oil drain empties into the timing cover so if visible there you after you take off timing cover, you could suck the drain clear (as opposed to blowing it.)

Some may suggest a little STP to thicken up the oil, but I don't think that will clear up a very visible leak. Maybe...

So if you do end up opening it up, I have just done all the learning you will do and can help.
There is lot of interesting stuff to learn and this forum is so helpful

Dennis
Click to expand...
I've pulled heads , inspected and cleaned carbon etc. with new seals job ( J.S. Teflon best ) , whole job done I one full day . Enjoyable . Don't be afraid with the proper tools .
As LAB said , your year predated seals , so if you decide to dive in you may need a machine shop for upgraded later guides if worn out .
Enjoy . From the Golf Coast Africa .
 
L.A.B. said:
Inlet valve guides with stem seals were not fitted until mid 1971 from serial 149670.
Click to expand...
Ok. So i read some things that lead me to that being a possibility. As in early commandos did not have intake valve stem seals. Is it possibly a worn guide or maybe just not draining quickly and pooling?
 
marshg246 said:
+1 on @B+Bogus

Make sure it's oil and not too rich smoke. In other words, if you're sure it's at less than 1/8 throttle, it's more likely the air screw in too far or wrong float height or sticking.

Do not run a Commando on the side-stand! The intake side oil drain is on the timing side so the intake area over-fills with oil on the drive side. Also if the float heights are not correct it is possible to cause over or under fueling although less common.

So, if it doesn't start on the 1st or 2nd kick and/or doesn't idle smoothly without blipping the throttle, when sitting upright, fix that before anything else.
Click to expand...
It’s oil smoke. I can smell the difference. Aside from color, one smells like raw gas, the other smells like wax. It’s oil. I don’t really “run” it on either stand. It’s upright between my legs when it’s running. I start it and shut it off between my legs. Unless i let it sit around a few weeks i can typically get it started first kick if i get the secret hand shake right, and once its warmed up it idles fine
 
Torontonian said:
I've pulled heads , inspected and cleaned carbon etc. with new seals job ( J.S. Teflon best ) , whole job done I one full day . Enjoyable . Don't be afraid with the proper tools .
As LAB said , your year predated seals , so if you decide to dive in you may need a machine shop for upgraded later guides if worn out .
Enjoy . From the Golf Coast Africa .
Click to expand...
It’s not a matter of being afraid… it’s more a matter of leaving well enough alone and not taking apart a strong running bike
 
Schwany said:
It's an old British motorcycle. My first reaction is just ride it if it is not smoking at cruise and it idles smoothly. If it is loading up smoking at idle and stalling you know it is more than likely carburetion. If it just smokes a little coming to a stop after a couple of miles, the sump may have been overly full from sitting a couple of weeks. Are you draining the sump after the bike sits for 3 weeks? I'm not sure if that year had a sump drain plug. If it does you could check to see if a lot of oil drains out of the sump after it has been sitting for weeks. Time for a reed valve crankcase PRV update maybe, and or one type or another anti-sump update.

Anywho, pulling the head off for a refresh is a PITA first time, but not turbo prop science. A borescope might tell you something before pulling the head off. Embrace the update snowball, or leave it basically stock. In stock trim a Norton works pretty good.
Click to expand...
I am pretty confident it’s oil, not smoke because of the smell and color. It idles fine and doesn’t load up. It does have a sump plug, however i do not drain it when it wet sumps. I just get it running and gently ride first few miles and assume the oil goes back? I know if it’s wet sumped and you start it and ride hard, it will push oil in the primary from crank case pressure. Will it not clean up if ridden gently for a few mins? I really noticed it last night after a 20 mile round trip, and the last 5 miles being ran hard
 
concours said:
have you done a compression test?
Post the results.
Add a teaspoon of oil after, re-test, post the results.

Also, IF you have someone you trust to ride your bike, and you follow it on another, OR, someone you know has observation skills and engine knowledge to follow you. Also, at night, with headlights behind you, smoke will show up in your mirror quite readily.
All to confirm/rule out smoke at cruise/accel.
Click to expand...
I have not done a compression test. It did not really catch my attention until last night. Good tip on the oil in cyls. If compression raises a good bit with oil that means rings or bore. Common airplane trick
 

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