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It's been very quiet in her lately so I'm guessing I'm the only one having an issue

I acquired a desmo head for my 450 recently. I probably should have dismantled it and examined everything in detail first but since it doesn't take long, I thought I'd pop it on and see how it performed. That was clearly a mistake - within a minute of running the plug fouls with oil. Is it possible that the valve guides are so badly worn that they could be responsible or should I be looking elsewhere. Seems incredible to me that worn guides could be responsible for that much oil ingress but has anyone seen similar behaviour?

Ian

Ian, the inlet valve guide has an internal groove in which sits an oil seal O ring, these eventually go rock hard and become ineffective, when the engine is cold (and the oil) the head will be pretty much flooded with oil and if there is any issue with the inlet valve guide oil will get sucked in to the cylinder to greater or lesser degrees. However the above mentioned O ring should not in itself be enough to drown the plug......so for me two things to look at....... is the inlet valve guide loose in the head? and has anyone been 'port tuning' the inlet port and broken through into the valve chamber (even just a little, check for welding or epoxy filler on the floor of the valve chamber)......this valve chamber 'breakthrough' is a common outcome with home tuners, either of these will let enough oil in to the cylinder to cause your problem. Frankly as an aside you were taking a bit of a risk trying an unknown 'desmo' head with out a stripdown, its a rare 'desmo' that won't at a minimum need the rocker clearances set (in my experience) I'm sure others will advise too. Cheers, George

Thanks George. Yes, I knew it was a risk but it was cheap enough to take the risk frankly and I was expecting to do some work. I’ll pull it off and take a look - it’ll give me something to do now that I’ve retired. I’m thinking valve guide issues myself but it just surprised me that it could foul a plug so quickly.

Ian

Hi Ian,

I can back-up George's comment - my 900 Desmo began doing this when it was about 35 years old and untouched. I would start it up and leave home and once the rear head filled with oil it would smoke and mis-fire until the oil became hot enough to drain from the head. Luckily it had the later top-hat guide seals and I was able to change them in situ, after which the problem went away.

Let us know what you find.

Cheers,

Colin

Thanks for the confirmation Colin. Will update when I’ve had a chance to take a look.

Ian

IanHood wrote:It's been very quiet in her lately so I'm guessing I'm the only one having an issue

I acquired a desmo head for my 450 recently. I probably should have dismantled it and examined everything in detail first but since it doesn't take long, I thought I'd pop it on and see how it performed. That was clearly a mistake - within a minute of running the plug fouls with oil. Is it possible that the valve guides are so badly worn that they could be responsible or should I be looking elsewhere. Seems incredible to me that worn guides could be responsible for that much oil ingress but has anyone seen similar behaviour?

Ian

Indeed it is quiet, but I check in most days for a look.
My little Ducati hasn't been out since NYD and is blocked in the garage by a partially dismantled Guzzi.
There are a few prep jobs to do on the Ducati when I can get at it.

I also have a pre-war Rudge. When I first had it and had learned how to start it, the plug was a bit prone to fouling, so I carried some spare ones on my short test rides. On one occasion I got through 4 whilst still a few miles from home - my partner was not pleased to be dragged out to bring another.
In my case it turned out to be excessive piston ring gaps letting a lot of oil "upstairs." It has open valve gear so I knew it was not oil coming down the guides.

That made me smile. I have two other bevels, a Darmah and a 750 GT and I carry spare plugs with me whenever I go out on them for 'just-in-case'

We clearly need some rocker box gazers.

So, I have the head off now and the valves and guides appear to be very good however I'm a bit concerned about the area at the circumference of the inlet valve (the bit adjacent to the exhaust valve). Does that look like something that could be allowing oil ingress? Have also attached pictures of the exhaust and inlet ports.

Regards

Ian

Hi Ian,

Something has obviously happened to that area of valve seat - perhaps a piece of valve has broken off in the past - but I don't think that it is anything to worry about. The exhaust port looks oily, but I find that all my bikes exhaust ports look like that these days; I'm blaming modern fuel, all that maize oil. Have you extracted the guide 'o' rings?


Colin