Blue smoke RH side when starting

Per

Per

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I have a 920 cc Commando that smokes blue on the right hand side when started. After a short time the smoke is gone, everything is fine while driving, but the RH plug is black. The valve guides are not worn and the valve stem seals are new and not loose. It is not related to wet sumping. I have removed the cylinder head, but can't find any obvious explanation for the problem.
The distance between the oil return hole from the RH intake valve and the cylinder liner is very narrow on the 920. Does anyone know if it is a common fault that the head gasket does not seal sufficiently around this hole so that oil leaks into the combustion chamber?
Does anyone have any suggestions?
 
There's a few different scenarios to check. As you said, it could be leaking between the pushrod tunnel and the cylinder at the head gasket. I use a flame ring style gasket on my 750 which seals fine, but always had trouble with the copper gaskets sealing well. I know there's some tricks and a sealant to seal the copper head gaskets better. You could probably do a search here and find that recommended process.

On a 750 there's a weep hole in the head on the timing side that aligns with the barrels and a hole in the head gasket to allow oil to drain back down to the crankcase. If that hole gets clogged sometimes the valves can be "under water" and suck oil down when the intake valve opens. It's on the right side and can be "poked clear" from the valve cover with piece of guitar wire. I don't know if the 920 version uses that hole. I've seen them with an external oil return line in the head too... Either way the same principle applies if that external oil return line gets clogged the intake valves can be underwater...

Maybe your intake valve oil seal has popped off the valve or has disintegrated. Both are known to cause oil consumption. worn exhaust valve guides tend to continue to burn oil rather than clear up as you ride, and also it stains the exhaust valve stem and the spark plug stays clean given that the burning oil is at the port, not in the cylinder..

I had a barrel that had a scratch from a circlip that popped out and rattled around for 100 yards until I shut it down and hauled the bike home. It would give a puff on start up, and when I rebuilt the top end some years later that cylinder's piston definitely had more carbon and unburned fuel staining on it, just from the scratch leaking tiny amounts of oil...

My guess would be the gasket, but you should see some staining from oil migration if that's the case, but then why would it go away when the bike heats up??

Anyway, I only answered your post to bring it back to the top. Maybe someone will have a more definitive answer...
 
What condition is the cylinder bore? has the bike got an air filter/s? have you done a compression test? If the head is as ok as you say it is, it is pointing towards the pistons/rings/cylinder. Any sort of vertical lines that can be seen down the bore equals burning oil to a degree. Can you still see the honing in the cylinder walls?
Did you buy the bike as an 920 or had it built by someone? Any photos of the cylinders when you had the head off?
 
o0norton0o said:
o0norton0o said:
There's a few different scenarios to check. As you said, it could be leaking between the pushrod tunnel and the cylinder at the head gasket. I use a flame ring style gasket on my 750 which seals fine, but always had trouble with the copper gaskets sealing well. I know there's some tricks and a sealant to seal the copper head gaskets better. You could probably do a search here and find that recommended process.

On a 750 there's a weep hole in the head on the timing side that aligns with the barrels and a hole in the head gasket to allow oil to drain back down to the crankcase. If that hole gets clogged sometimes the valves can be "under water" and suck oil down when the intake valve opens. It's on the right side and can be "poked clear" from the valve cover with piece of guitar wire. I don't know if the 920 version uses that hole. I've seen them with an external oil return line in the head too... Either way the same principle applies if that external oil return line gets clogged the intake valves can be underwater...

Maybe your intake valve oil seal has popped off the valve or has disintegrated. Both are known to cause oil consumption. worn exhaust valve guides tend to continue to burn oil rather than clear up as you ride, and also it stains the exhaust valve stem and the spark plug stays clean given that the burning oil is at the port, not in the cylinder..

I had a barrel that had a scratch from a circlip that popped out and rattled around for 100 yards until I shut it down and hauled the bike home. It would give a puff on start up, and when I rebuilt the top end some years later that cylinder's piston definitely had more carbon and unburned fuel staining on it, just from the scratch leaking tiny amounts of oil...

My guess would be the gasket, but you should see some staining from oil migration if that's the case, but then why would it go away when the bike heats up??

Anyway, I only answered your post to bring it back to the top. Maybe someone will have a more definitive answer...
Click to expand...


Anyway, I only answered your post to bring it back to the top. Maybe someone will have a more definitive answer...
Click to expand...
Thank you for your answer. I have checked the places that most often cause blue smoke. If the problem is in the cylinder, the smoke will not stop after 20 - 30 seconds. The same applies to worn valve guides. The valve stem seals are in place and in good shape. The oil return holes are the same for both 750 and 850/920. They are checked and open. There is no oil leakage from the head gasket outwards. It is frustrating not to find the problem. I will see if anyone else comes up with any good suggestions.
 
dobba99 said:
What condition is the cylinder bore? has the bike got an air filter/s? have you done a compression test? If the head is as ok as you say it is, it is pointing towards the pistons/rings/cylinder. Any sort of vertical lines that can be seen down the bore equals burning oil to a degree. Can you still see the honing in the cylinder walls?
Did you buy the bike as an 920 or had it built by someone? Any photos of the cylinders when you had the head off?
Click to expand...
Thank you for your reply. The cylinder bore is ok. No scrathes or vertical lines. I think that problems in the cylinder would result in blue smoke while driving as well. Not just at start up. I built the bike many years ago. In recent years it has been ridden very little.
 
You could try disconnecting the oil feed to the head
Then you'd be certain it was not the rings
You can run for sometime without oil to the head
 
baz said:
You could try disconnecting the oil feed to the head
Then you'd be certain it was not the rings
You can run for sometime without oil to the head
Click to expand...

I have disassembled the cylinder head in an attempt to find the cause of the problem. So far I can't find any obvious reason. If I reinstall the head and the problem still exists, one solution is to disconnect the oil hose to the rockers in the continuation of troubleshooting. Thank you.
 
sounds like a valve guide or stem seal... where a little is leaking when sitting causing the brief smoke at start up, but then when that's burnt off the seepage is negligible.

point being everything checks out as "good" - so its something you think is good but is not- and the above seems to be at least a cause
 

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