.I've done the water drip deal and it does seem to help. In my case, I
ran a hose from a water jug and put it on he carburetor in place of the
vacume advance hose. I ran the car that way letting it draw the water in
and it did loosen up a lot of deposits. (Its hard to keep them running
at idle when you are dripping water into the carb so I did it this way
instead).
The high RPM cleanout is a lot more fun though.
Other causes of the dieseling would include anything that allows too much
air into the system at idle. I've seen a vacume leak cause it. If the
car is equipped with an idle speed solenoid it has to work and it has to
be adjusted correctly or it might be the problem. Idle speed set too
high will do it. If the ignition timing is too retarded and someone set
the idle speed to the correct RPM, the throttle will be open more than
its supposed to be at that engine speed (there's not as much advance so
it has to burn more air/fuel to make the same speed) and that can do it.
Ditto for incorrect dwell etc... Anything that's wrong if the throttle
position was adjusted to compensate.
Wrong heat range on spark plugs. Combustion chamber hot spots and/or
deposits. Anything that raises the temprature in the cylinder head area
above what its supposed to be can contribute. Plain old crummy gas can
also do it.
For what its worth, it gets pretty hot down here in Fla. (102+ just two
weeks ago) and every time I have seen a Chrysler big block start to
diesel it has been because of too much air getting in for one reason or
another.
An INSTANT method of stoping the engine when you turn the key off that
usually works is to just leave it in Drive when you shut it off then put
it in Park. The extra drag from the transmission is usually enough to
stop the engine. That won't fix the problem but it will (most likely)
give you a way to keep the engine from hurting itself while you do find
the problem and fix it. I Strongly Recommend that you start doing that
right away. I had one that had a vary bad case of dieseling that I
traced to an intake manifold leak. It was a 440 with a Carter AVS. When
it dieseled, it would spit fuel out the top of the carbuteor. I was
asked to look at it when the owner (I bought it later) shut it off and it
blew the metering rods and metering rod pistons completely out of the
carburetor. They just aren't made to run backwards real well!
A vacume gauge, a timing light and a dwell/tach along with a shop manual
or any good "Trouble Shooting" guide written for engines of he same
general age as yours should show you the problem pretty quickly.
I remember in the early 70's Johnny Carson did a monologue wherein he
described what he did after work. It included driving home, pulling into
the garage, turning off the car, having dinner & so forth all the way
through to the next morning when he just got into the car and drove off
because the engine still hadn't shut off from the evening before. It
was funny at the time.
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