Give it a good tuneup with the timimg set properly and to the correct
spec for its year, the idle speed set correctly and try spark plugs a
range or two colder. Then put a vacume gauge on it, warm it up and set
the idle mixture to get the smoothest idle, highest vacume and idle
speed. Reset the idle to spec if that changed anything then see what the
vacume gauge says. If the engine vacume at idle is significantly lower
than it should be after all that is set correctly, its probably a vacume
leak or some similar problem that lets too much air into the system.
I'd put a new hose on the PCV valve if it is at all hardened just to rule
that out and I'd disconnect and plug every other vacume hose and fitting
on the engine (except the vacume gauge). Run it at idle again and see
what it says. If the reading is pretty much up to spec, one of those
hoses leaked or goes to something that leaks. You can find out which one
buy connecting them one at a time until you get to the one that causes
the reading to drop. If the reading is really low like 10 or 12 inches,
its probably something like a very loose/leaking carburetor flange or the
intake manifold gasket.
I grew up around these engines and have seen several that dieseled at one
time or another. It does get hot down here in Florida and heat does
contribute to the problem but every time one of them dieseled, too much
air turned out to be the problem.
Good luck with it and please do keep the list informed once you have the
problem licked.
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