slant six 170 plug foulfoul

I'm a little on the fence about this what we're seeing on these plugs, actually. Could be rich running, could also be oil getting into the combustion chamber via any of the usual suspects (valve guides/seals, rings). If the black gunk on the plugs is completely dry and fluffy/powdery, that's fuel -- too rich. If the gunk is at all "wet" looking, and it's more gummy than powdery, that's oil. Severe cases of either are obvious; less-severe cases like what you're showing us aren't as obvious.