Plug rust

I'd start it up to make sure it runs and has no other issues, then do a compression test.

You could also have a head gasket issue on that side causing the problem and if nothing else the compression test will give you some info on the engine condition.

If those plugs are arranged the same as they are in the engine, the porcelain on #5 looks quite a bit cleaner than the others which could be from water too.

So..........run it, check things and see what issues you might have and go from there is what I'd do.

Sunday's work went well overall on the 79 Southwind / 351 -- flushed out all the cooling passages and found bark fragments blocking about half of the trans cooler along with the normal mud and gunk. Replaced with automotive oil sender with a marine unit. Pulled the plugs, squirted in some oil, primed the oil system (holds 40psi) and turned her over a bit with a ratchet. Just need to get the carb on her and hopefully I've still got spark. I'm getting close!

So the plugs were gapped somewhere around .017 and were too tight for my trusty autozone plug coin to fit in the lowest setting. Odd -- not sure what that was about... Gapped to .035 and reinstalled -- but was concerned to see corrosion on #6.

I've seen riser gasket talk in a few places on the site -- I'm guessing that's the likely culprit? Are there any other potential causes maybe relating to the cylinders' 10-year fuel soak?