> I'm still experiencing running on, or dieseling, when
> I go to shut off the engine.  It only happens when it
> is at operating temperature.  When I shut it off when
> it's not up to temperature, it shuts off ok.  I have
> verified the timing, mixture, point and plug gap, none
> of ythat seems to make a difference at all.  The idle
> speed is as low as it can go (about 900 rpm).
As dnaniderf suggested, pulling a plug lets you see a little piece of the 
inside of the combustion chamber. Lean can also do this, so double-check the 
mixture hot.
Some vintages of MGB like my 1973 had a solenoid valve that opened when you 
turned off the ignition so that it would allow additional air into the 
intake and limit the run-on.
David Lieb 
 
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