Could just be a build-up of carbon deposits on the valves and cylinder
heads. Some modern petrols, and petrol additives you can buy at motor
factors, claim to clean this, but if you've got a major build-up it
might take a long time to clean it up! It's possible you'll have to
remove the head and de-coke it properly.
Timing shouldn't directly cause running on - after all, when you turn
the engine off, the spark plugs aren't sparking at all, whatever the
timing is set to. The reason timing could be a cause is that advanced
timing can cause the engine to over-heat, and an over-hot engine can
"diesel". But if your timing is that far advanced you should also
notice pinking when accelerating hard from low revs - if you don't, then
timing isn't your problem. There's a whole bunch of things that could
be causing over-heating of the cylinder head, but I'd want to be sure
that overheating was the cause of your dieseling before you diagnose the
cause of the overheating.
Finally, you can buy ant-run-on valves, that open to allow a whole lot
of air into the intake plenum when you turn the ignition off, which will
prevent running on. Of course, this only treats the symptom, not the
cause, but if the engine is running fine otherwise that may be enough.
Richard & Sammy ('73 Black Tulip BGT)
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