Classic/Antique Car Repair: 1966 Mustang Running after key removed, combustion chambers, battery cables

Question
QUESTION: I noticed you responded to a similar thread earlier this month, but I need some clarity please.  My car has a 302 with a brand new external voltage regulator, coil, solenoid, cables, battery, starter, ignition, etc.. Car has been running excellent for past couple of weeks.  Last night, out of the blue, it wouldn't shut off even after key was removed.  I disconnected main power and it still ran.  Then I disconnected both battery cables and it still ran.  I then disconnected external voltage regulator and it finally turned off.  Put everything back together, fired it up and it ran and turned off perfectly.  I've since started it and turned it off successfully about 8 times.  What could it have been?  Is it a sign of things to come?  Any help would be appreciated.  Thanks in advance.

ANSWER: Sounds to me like it was running on, or "dieseling". This happens when there are carbon deposits in the combustion chambers that heat up and act like spark plugs, so you don't need any electrical supply to cause combustion--these white-hot bits of carbon will keep the cylinders firing, which continue to heat the carbon, etc. Another symptom is that it seldom fires on all cylinders when this happens, so it will run ROUGH when dieseling.

The fact that you disconnected the regulator and it stopped, is probably a coincidence.

You should check the timing, and possibly advance it a bit, run good gas (87 octane may be too low) and if these ideas don't work, have a decarbonization done.

--Paul

---------- FOLLOW-UP ----------

QUESTION: Wow.  Quick response.  Thanks.  I thought about dieseling, but it wasn't running rough.  In fact, it was very smooth and ran for over 5 minutes.  I could even accelerate without the key.  May still be what you described, but thought I'd give you some more information to consider.

Answer
Next thing I'd consider, Troy, would be a harness short between any battery lead (the thick one from the VR is your best bet) and alternator output and/or coil hot.

You've got current being produced by the alternator bypassing the switch and exciting the coil. Keep that in mind and you should be able to find the fault.

--Paul