only 1 black sooty spark plug

I removed all 4 spark plugs on the wife's Fiat Punto 1242cc 8valve 1999 60s Mk1 single point injection.
All 3 had right colour. 1 plug (on number 3 cylinder I think) was black and
sooty buy dry, as if too rich mixture. - anyone know what this denotes and what to look for and can it be an extensive and expensive problem with the engine?
The engine is idling a bit lumpy (like its only running on 3 cylinders instead of 4) but when going at speeds of 40,50,60mph is as smooth as anything and not misfiring or any thing.
Any ideas please?
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Might be worth doing a compression check, this should show up a head gasket or ring failure. If all cylinders are same within a few percent, move onto an emission check to see if that throws anything up. There are several sensors, anyone of which can cause overfuelling, plus the HT generation and plug leads. Or just clean/replace the plugs and have another look in a months time. A lumpy idle is not uncommon and the cause often elusive unless you find a specialist, with expensive diagnostic equipment, and more importantly the know how to use it correctly.
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