Troubleshooting oil-fouled plug on 69 350 mk3 desmo

[neal69duc350d]

Hi all,

Started having a problem where the spark plug would foul after about 30 minutes of riding. Engine would cut off, could not restart, oily/sooty appearance on plug (but not saturated with oil, just kind of shiny). Replacing the plug and the bike would fire right back up. What's the process to identify whether this is from the top end - valves - as opposed to bottom end - piston?

Thanks,
Neal.

[blethermaskite]

I would be firstly suspicious of the ignition system......are you on points or electronic ignition?, is the engine actually burning oil ......is the exhaust putting out blue smoke when you rev up the engine with the bike stationary? If it actually is burning oil a hotter spark plug should effect a temporary stop to the plug fouling. Stick your finger into the end of the silencer and see if it comes out black and slimy. What's the compression like can you stand on the kickstart lever on compression for a few seconds? In my experience leaky valve guides will let the engine burn a bit of oil and give a light blue haze in the exhaust, but shouldn't foul the spark plug (unless you are running a colder plug than recommended) usually proper oil burning is a piston/ring/cylinder bore issue.....frequently broken or stuck rings.
I would still have a good look at your ignition system first especially if the signs of actual oil burning are not present.
Cheers,
George

[neal69duc350d]

Thanks George. Should have included this: points ignition and NGK B6HS plug. The exhaust smokes at idle and when revved. Not heavy amounts but enough that it is visible and blue tinged. The silencer is black and sooty, but not slimy. Good compression -- can get the PSI when I have more time tomorrow to run a compression test.

[Ventodue]

The exhaust smokes at idle and when revved. Not heavy amounts but enough that it is visible and blue tinged. The silencer is black and sooty, but not slimy. Good compression -- can get the PSI when I have more time tomorrow to run a compression test.

Blue smoke at idle = (normally) leaky valve guides.

That's why your compression seems good. If the rings were the cause, it wouldn't. You'll probably find that the oil consumption isn't that high - if the rings are worn, it is.

Might just be the O-ring on the guides; or could be worn guides. Probably best to leave as is for now and put right come winter time. Not a big job, just needs a certain amount of expertise/care.

[ranton_rambler]

I had my guides K-lined rather than replaced. You need to find someone locally who has the tooling for the correct size valve stems, but is less intrusive surgery than replacing guides. Everything stays aligned so need to recut seats. Obviously needs the stems to be in good condition.

[Duccout]

This also might be a clue: my 900 Desmo which when it was thirty years old and untouched from new, began to have a problem just after starting from cold, I would get a mile down the road onto a main road and when I tried to accelerate the engine would smoke and misfire and then clear. When I investigated, the valve guide seals were hard and were not gripping the valve stems.

Colin.

[neal69duc350d]

It really isn't burning oil, so I think I'll investigate the valves come winter. Thanks for the input.

So I can continue to ride the bike while the weather is nice, would going to a B5HS bandaid this issue as George advised? I hate to be replacing the plug every time I ride since B6HS aren't exactly easy to come by around me.

[Bevel bob]

You might try an ancient plug (if you can find one) as some modern plugs appear to have little hard glaze on the insulator which allows them to absorb the carbon into the insulator and then it resists the self cleaning action that should occur. No one is actually making plugs now that are purposed to cope with our old tech engines and carbs. Lean burn is king.

[ducwiz]

I had my guides K-lined rather than replaced. You need to find someone locally who has the tooling for the correct size valve stems, but is less intrusive surgery than replacing guides. Everything stays aligned so need to recut seats. Obviously needs the stems to be in good condition.

How do you "K-line" the desmo valve guides while preserving the O-ring grooves?

Hans

[ranton_rambler]

I had my guides K-lined rather than replaced. You need to find someone locally who has the tooling for the correct size valve stems, but is less intrusive surgery than replacing guides. Everything stays aligned so need to recut seats. Obviously needs the stems to be in good condition.

How do you "K-line" the desmo valve guides while preserving the O-ring grooves?

Hans

Sorry, mine is a spring head not Desmo. No seals.