Laura,
Usually with 'run on' it is a combination of little things that
will help aggravate the problem.
High idle speeds, high compression, carbon build, correct
spark plug range, etc. up all add up to cause some issues.
What is happening is the engine is igniting the fuel mixture
without spark (dieseling or running on). Carbon build up can
have enough "glow spots" to help ignite the fuel. Also with
a build up of carbon, it will also raise the compression
a little more. The higher the compression, the compressed
air/fuel mixture temperature will reach higher temperatures
allowing self ignition.
Of those, the easiest to address is engine idle speed. The
idle speed should be around 650 rpm. When you shut off the
engine at that rpm, it will stop nicely.
In really bad cases you need to "de-coke" the cylinder head.
It involves removing the head and using a wooden stick (doesn't
scratch the piston tops, nor the cylinder head) to remove the
built up carbon.
You'll need to do a full tune up, and valve adjust first.
If the carbs are in great shape (no worn throttle shafts),
and the idle mixture set up correctly. Then it is pretty
simple to maintain a low idle speed.
Shut off the key... engine stops!
Having said all that... with worn throttle shafts on my
68 2000... I would just leave the car in second, foot firmly
on the back, and release the clutch as I shut off the engine.
No run on with the extra load.
Cheers,
Tom Walter
Austin, TX
P.S. Years back when I worked in a gas station, I was given
the assignment to "Italian Tune Up" a corvair that would
develop a run on problem. Owner never get the revs up high
enough... a few high load (full speed) engine runs down the
highway would blow out the accumulated carbon. Engine ran
much better after the "mini tune up!"
P.P.S. Check you choke cable and that the "jets" pop back
up into the carb body. Leaving the choke on too long will
tend to build up the carbon deposits in the engine.
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