DA, I have a smoking problem on 1 cylinder of my Mk III, I'mcertain this is Valve related. I'm not sure if this is just valve guide seal, or whether the valve guide also needs attenton. Much to my surprise theguides were replaced 20k miles ago - time has flown by!
The head is now off the bike with valves still in situ,what is the best way to check the guide for wear? Could I pour petrol in via inlet rocker box & leave for a while to see if it leaks through into the combustion chamber?
A de-coke is also required, what is the best method to use in a domestic garage,I know not to use a screwdriver dare I use a dremel/drill & a brass wire brush or similar?
Cheers, Mike (mk III)
I should have thought that…
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You have two potential iss…
You have two potential issues with the valves. The play in the valve guides and the seal of the valve seat. The only way to measure the play in the valve stems is to dismantle and measure with micrometer and bore gauge. These can be bought quite cheaply now and whilst they may not be the best quality they will suffice. The workshop manual will give the figures for sizing. I seem to recall about 0.002" was the clearance but not totally sure.
Before you dismantle the valve gear invert the head and fill the combustion chambers with diesel or paraffin and let it soak. Try scraping the carbon off with a sharpened handle of a toothbrush. I think a dremel and brass brush is acceptable provided you don't go at it like a mad thing.
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After parafin or diesel,I…
After parafin or diesel,I use scotch bright and elbow grease
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Due to the rocker geometry…
Due to the rocker geometry, Commando valve stem tips are often slightly damaged and the edge may need stoning slightly to remove any burr before dropping the valve through the guide.
In order for it to smoke badly, assuming intact seals, there would need to be quite a lot of wear and this could be felt by removing springs, lifting the valve slightly off its seat and rocking it. There should be nothing more than just perceptible movement. If there is, and you're not intending to do the job yourself then you'll need a specialist anyway.
Mk3 heads can be quite thin between guide and inlet tract and the guides can also become loose in the head which will require oversize guides and some very careful assembly to avoid further damage.
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I should have thought that if you pop the valve springs off, you wouldthen be able tofeel any excessive play, valve to guide. You could then mike up the valves and guidesto check them for wear. However, I guess you could do what you suggest first, but while it is apart.....
As for the decoke, which will require valve removal anyway, I always used to use a penny; I guess it will have to be a 2p piece these days; that's inflation for ya.... I certainlywouldn't trust a brass brush on a drill, as aluminium is generally softer than brass, so I know which one would win - ouch.
Ian