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One sooty plug


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I have one sooty plug on number three cylinder and the others all the correct colour, car runs very well. 

I'm guessing worn Rings/Bores or worn valve guides?

I haven't compression tested yet

What are the thoughts?

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could just be a duff spark plug       black   soot is normally a rich setting not a piston/ ring wear symptom

 modern  plugs do have problems with carb fueled cars 

do check the choke is fully returning the jets up to their adjust nut and there is no sticking.

check valve clearances 

ht leads last years and few give any problems    swap them round  as suggested to prove 




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27 minutes ago, Stratton Jimmer said:

As a first and easy check, I would swap the sooty plug with number 1 and see if the swapped plug soots up and the formerly sooty plug turns clean. Then go for a compression test.

Yes I swapped the plug, so not there, not thought about swapping the ignition lead though

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