With thanks and acknowledgement to those that contributed on the FB site, I thought I would also ask those gathered here - any ideas on why these plugs look the way they do? Single carb car - compression test gave 160psi on 3 cyls and 165psi on 1 cyl. Leakdown test provided 4-5% on 3 cyls and 7% on one cylinder.

Thanks.

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'68 Deluxe w/Metro engine

Have you checked for inlet manifold leaks ?
Cyl 3 & 4 leaner than 1 & 2
just an option to look at .

Thanks Slacko. That came up on the FB post. Hard to imagine where a leak could exist, but I may just have to pull the manifold and check for flatness. I guess there is a very slight chance that the water cooling passage has a corroded through, but it hasn't had water in it since I've owned the engine. And of course the manifold gasket itself... Cheers.

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'68 Deluxe w/Metro engine

easy to check
with the engine running pour water or wd40 over the inlet branch where it bolts to the head and if you have a leak the engine tone will change , water will probably kill it and wd40 may make it speed up , if so a new gasket could fix this

Matt,

I've had cases in the past where the inlet manifold does not sit square or flat on the head due to extractors. i.e. not enough clearance.
Just a thought.

Pete

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1967 Cooper S - new front bearings to do.
1965 Cooper S shell - Slow progress. No time or money!
1966 Deluxe- next rustoration!
Mk 2 & XJ6 Jags. Less said the better.

Have you discounted weak spark. Bad HT leads or spark tracking at the cap.

gtogreen1969 - I haven't, but it is running quite well. Could I have weak spark to discolour the plugs yet it runs ok?
I'm assuming on this suggestion we would be looking at weak spark on 1&2? (ie tracking between 1&2?)
Thanks for the suggestion.

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'68 Deluxe w/Metro engine

Chuck an ohmmeter on the leads. Anything around 2k ohms is good.