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Oily Plug

S

Slowhand

Just got it firing!
Joined
May 13, 2014
Messages
1
Problem with a 1991 1000S, Le Mans engine, 35,000 miles from new. Starts and runs perfectly, hot or cold.
But in 50 miles or so, it loses the R/H cylinder and appears to have oiled the plug (BP7ES) Change plug and away it goes again for about 50 miles, by which time the new plug is shiny black, oily and not sparking under load. Swapping for a softer (hotter) plug BP6ES helps, but it ain't right.
Rings or guides would be the obvious diagnosis, but without removing a head, I can detect no more than usual side play in the valve stem/guide clearance.
Compressions are 175 psi hot on both cylinders, though it used to be 190 psi.
No noises from either cylinder to suggest a broken ring.
Ignition system is standard points with Dyna coils, and timing and advance are spot on.
Carburettors (40mm) and float levels are fine, though I might suspect the choke piston seals could produce a rich mixture at full throttle openings.
There is no excessive smoke from the R/H exhaust silencer, and the plug insulators come out a light tan colour, as they should. But the R/H plug goes black in 50 miles and stops firing. A colourtune test was inconclusive.
The engine has never been apart from new and I am reluctant to strip the heads unnecessarily.
Any suggestions or diagnoses would be very welcome.

Thanks.
 
Before we go into a tear down. First make sure oil the drain hole for that cylinder hasn't become blocked and the vent line is also not blocked.

If they are open, then it sounds like the engine is tight with that compression so a leak down test won't tell you anything. All I can think of is worn valve guide, intake side. This would allow oil in even with good compression. If you pull the carb and manifold can you see oil on the intake valve? If a guide issue I do recommend using K-line inserts in the valve guides instead of replacement of the guides. Only select machine shops use that process. The site is here: http://www.sbintl.com/ From there you can contact them or a distributor to find a local shop.
 
Hi,

just a thought , but maybe the rh coil is failing after getting hot ? easy to swap coils over and see if the problem occurs on the left side now ? Check earths and connectors whilst you are are there. I would also look at re newing the o rings on your chokes , or the plungers , to be sure , to be sure , dave
 
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