Just One Oily Plug and a miss-fire

So last summer I had the XK120 on rolling road and set up by a guy who knows his stuff here in the UK, so no issues there. Now the #6 plug (the one nearest the firewall) fouls up after 20 miles or so; so badly that the car mis-fires / runs on 5 only. The other plugs are fine color wise.

Early 2018 the engine had a rebuild and there has has been a bit of valve noise and was agreed that would let car settle in for a bit before adjusting. So would an “off” valve clearance be enough to allow 1 cylinder to run extra rich ? Or should I be looking for something else too / instead off ?

Collecting expertise and thoughts before collecting spanners :slight_smile:

First principles: check the clearances. That said, it would take a valve seal being bad to cause oily plugs (along with a few other things) over a misadjusted valve.

can you post a close-up picture of the plug?

Just for clarity the plug next to the firewall is actually No1 and is it oil or soot (unlikely to be one cylinder rich)

I more often see a plug that clears up after 20 miles. I wonder if your ring gaps are not staggered as they should be. Some say this can change in use.

The rings do migrate around the piston, which is desirable. Early engines had no valve seals, but this recent rebuild may have them…mine now does.

I would suspect a tappet guide that may have come loose. I haven’t had it happen but they are known to do this. It may have loosened up after the re-build. It could be letting excess oil around the valve stem. Isolate which side is making the noise and pull the cam-cover for a look.

Ive just cleaned it up so can get a few miles on in good weather :slight_smile: but hopefuly a dozen miles and it will be nice and black so can post a picture on Sunday night

Thanks to all for the thoughts…

Tweety had a cylinder which would oil foul: I ran Champion N12Ys on it, and that cured it.

Till it got rebuilt!

That is a loud noise before it allows more oil to run down the valve guide if that’s a possibility. When they do come loose, the camshaft knocks them to pieces audibly, valve gaps should be nowhere near that. Simple to find out by removing the cover and also looking at the valve clearance while at it…

that is pretty quick to foul…when you look at the plug is it so crusty with black…that it connects across the " gap.".? .what plug type and number, and gap are you running?..With a sports coil…try NGK BBP5ES…and .028. Carry a spare pre gapped plug and the R and R tool…for a 2 minute replacement on the roadside. and yes…#1 at firewall, #6 at front. Check plug wires for resistance…many new wires have way too much…designed for modern cars. Use no resistance wires. FYI plug wires on dizzy cap in fire order 1-5-3-6-2-4 ( 6-2-4-1-5-3 is same) counter-clock. Be sure timing is correct…add a little Techron or Gumout with PEA fuel cleaner to fuel, you want as optimal clean burn as possible. (these are not fixes to the actual cause, …but perhaps will aid to not foul so fast until a fix can be done…and IMHO these help on any XK. Nick

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Thanks for the suggestions all. After some distractions with other projects (etype, garden, race car) - checks on the mechanical side reveal no problems and am now trialing some BP5ES (was on 6’s) with .028 gaps (thanks Nick and a subsequent post on plugs / service bulletins). So far first 140 or so miles with no issues and am going to start a “photo diary” with plugs 1&2 and 5&6 doing side by side comparison for a few more runs - probably no visible difference but there you go !

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did you mean…THERE is a loud noise…? yes if a follower guide comes loose and moves up…the cam contacts it…rapidly chewing the rim to bits…with a lot of unpleasant noise, now metal bits in the oil…and even more bad noises very soon. Nick