She's smoking

My 71 750 Commando's right cylinder smokes for less than a minute upon start up. After that it stops. Oil consumption not a problem. I am thinking valve seal. Engine rebuilt was done about 2K miles or less ago. Is it feasible to replace the valve seal without removing the head?

I do believe the guide may be a non stock item, in that it was originally an oversize one, but had to be replaced when the original oversized one starting sliding up and down in the head and pumping oil in the cylinder. Since the smoking stops after about a minute and the engine has been fine up until now, I am thinking the guide is not moving, but the seal is leaking.

Any thoughts would be appreciated.

Ludwig showed how to do this once, try a search on his username plus valve, you can use rope in the combustion chamber to hold the valve up or use air pressure.

All of my and my bud Wes's bikes and scooters good through this worn ring phase, smokes when cold for min. then fine till next day cold start, till rings smoking even after good long warm ups. Prior can go 1000 miles or more before mis firing and embarrassment on the road in public. Head expands with heat so tend to smoke more as ya go and more on let off than getty ups. i have sworn off leak down testing but for TDC only, if ya think ya man enough to keep in on TDC with pressure on jug. My way now would be just pull barrel and check ring gap and if ok then look into the head.

My bike possesses similar characteristics.

When I start it up, after it has been standing for a few weeks - and it has wet sumped - it smokes for about a minute from the right pipe only. Thereafter it runs clean. If I restart it again after a few days, no smoke.

What I fail to understand, is why it only smokes on the right.

Any ideas?

Hm wet sump bore splash would smoke before oil pressure hit head. Might try a diagnostic experiment, drain sump before next cold start up and see what happens. If takes longer to smoke then likely just rings, unless bores got scored then another decision point, ie: enough meat to light hone or next bore up, ugh. If blow by gets too bad it can ruin the oil tightness to point just rings stopping it and normal re-torque don't really seal again, ugh.

Smoking on startup is usually rings, not valve guides.

Rings smoke on acceleration, valve guides on startup. ?

A compression test may show if rings give low compression on one side.

I once replaced a valve guide seal on a rebuilt 850 not long after.
It seemed to have 'fallen off', and the spark plug on that side was always dirty.
Replacing it fixed all that.
Not simple to do with the head on, I just took it off and replaced it.
It didn't smoke much on startup though, more just used and burnt some oil on that side.

You checked that the oil drain on that side is all OK, no pools of oil up there when its stopped ?

Thank you for the replies.

There are a few differences of opinion here...

"Rings smoke on acceleration, valve guides on startup. ?

Smoking on startup is usually rings, not valve guides."

It only smokes when started up and when the sump is full of oil. It smokes only on the right, which infers either the rings on that cylinder are less effective, or there is something unique about the right cylinder resulting in more oil thrown at it. This appears unlikely. A stand on kickstarter compression test (very unscientific) suggests compression is similar on both cylinders.

I will check next time if some of the torrent of oil returning to the tank ends up in the airbox. (850 standard Mk 2). From memory, I cannot remember if there is any bias to the tank breather in the airbox.

I am struggling to see how guides could have an effect - given the unique set of circumstances in which it occurs -or am I missing something?





You checked that the oil drain on that side is all OK, no pools of oil up there when its stopped ?

Are you referring to pols around the inlet guides?

When my rings started to give up the ghost the left side started smoking on acceleration first, the left side is further away from the oil pump so wears slightly quicker. So oil on the right side on start up sounds like guides/guide seal to me.

Thanks for the very quick response.


If the problem is with guides, how does the wet sump oil end up in the head? This is what puzzles me.

on cold stars with some normal blipping into 2000's it can take 20+ sec for oil pressure to hit head then a bit more to collect enough to feed a guide leak and then generally only the intake side as exhaust guide should not see any low pressure from chamber. Of course not unheard of to have two separate issues doing the same thing, Three if gasket sealing at tunnels or oil drain hole involved. Look for stuff where there is light enough to find it and fix the easier cheapest quickest thing first, ie: rings-bores, then see what happens next.

If you do do any other diagnostic device assisted tests please please please video the educational event for our ongoing vintage slap stick entertainment.

I have a 72 that does the same, I can not think why...bad drain back from the rh valve/rocker pocket? Had a '71 750 Dunstall, back in the day, that never did it...