Dealing with dieseling-run on

[Kern Dog]

After a man has spent some time in late model stuff, you forget about some of the picadillies we deal with with old cars.
One of them is the tendency for an engine to sputter and snort when you shut it off. I am dealing with that now.
The car....



440 based 495 with dished pistons shaped like this:



The flat portion sits at zero deck to encourage quench. The stepped down section is dished to lower compression.
I'm at 9.8 to 1 with the Edelbrock aluminum heads. I use a Demon 850 vacuum secondary 4 barrel on an Edelbrock RPM intake. I have NGK BPKR7 spark plugs and electronic ignition.
I had a big Lunati cam in it but now have a milder hydraulic roller. In both setups, I've had the dreaded dieseling when I shut it down.
Here are my findings:


Cold start, I let it run a couple minutes and shut it down. No run-on.
I got it to about 120 degrees and tried again. It shut off like normal.
I got it to 180 and tried once more. Still shut down and stayed off, no run-on.
I did not drive anywhere. It is an 85 degree day and the car was not driven to spread heat around everything. The fuel is not heated from driving either. This was meant to just see how sensitive it might be and I had to start somewhere.
I have read about this subject and seen comments related to how fuel dripping or a throttle blade opening should not cause this condition, something has to ignite the fuel. This is where deposits or hot spots would contribute to this. With the fresh valve job and cleaning that I did a couple of months ago during the cam swap, the deposits are minimal or non existent. Some comments online allude to low quality gasoline being a possible issue even if the car doesn't detonate with it. I would like to know how that can be. I sure am no chemist or engineer but it just seems like if it runs so good on this gas, why would it run-on after shut down?
I just ran it up to 2000 rpms and shut it down while holding the throttle open and it shut right off. The temp was 180. I have not tried that before. I expected it to ka-lunk...ka-lunk...ka-lunk like it has done before.
Maybe with the temperature up to the usual 190-200 that will change.
More to come.

[Kern Dog]

Today I drove around a bit but still have not pinpointed what causes this run-on. I forgot to mention, it idles at 800 RPMs.

* Cold start and drive. 25 minutes on the road to a friends place, 95 degree day and the engine got up to temp and stayed there. 89 octane ARCO gasoline. On shut down, it had a short 1 second **snort** and then was done.
* Start up and drive to gas station, added 6 gallons of 91 octane ARCO fuel. Small dieseling when shutting it off to fuel up. I drove another 1/2 hour to a speed shop.



Same thing but the 1/2 hour on higher octane gas made no difference. It snorted when I shut it down.
Back on the road, another 1/2 hour to 45 minutes back to the friends place. I let the engine idle 10 seconds after stopping, then twisted the key. It dieseled but as it started to, I turned the key to RUN, then back to OFF. The dieseling stopped.
I did the same thing later when I drove it home.
Keep in mind, the car does not knock or detonate even with the 89 octane fuel.
I know that I could just shut it down in gear and it won't run on but to me, that is a crutch. It should just shut off and stay off.
Since the pistons were cleaned when I had the heads off in February/March, there should be no hot spots there. I had the heads fully apart for cleaning and lapping the valves then so there are no deposits there either. What is causing this?
It only does it once plenty of heat has made it's way into the whole car. It did not diesel when sitting in the shaded garage even at a 180 degree temperature on the gauge. This car does not overheat. It runs fine in hot weather. I've driven it in 110 degree summer heat and it does not boil over or detonate. The Air/Fuel meter runs at 13.8 to 14.0 at cruise speed which is almost perfect for a 10% ethanol mix.
At FBBO, guys have suggested trying Lucas fuel treatment or gas from a better station. I have always figured that if I was not knocking, the gas must be good enough. Some didn't think so but I'd need to know why they think that.
Things left to try:
*Throttle blade opening. Yeah, if the throttle blades are opened too far at idle, maybe the engine slowing after shut down can still draw fuel into the chambers....but something has to be igniting it, right? Could warm/hot pistons alone be enough?
If I am able to close the throttle blades more, I'd need a way to bring the idle back up to the 800 RPM number that I have now. This can be accomplished by increasing the timing at idle either by twisting the distributor or switching to manifold vacuum to the vacuum advance.
*Spark plugs with a colder heat range. If the current plugs are retaining too much heat, colder plugs may work since they would not serve as a heat source to ignite the fuel.
More to come.

[70 sublime]

My last 69 Charger had a 383 out of a 71 Newport in it
Just a plain 383 with 2 barrel carb on it
Engine had 75 000 miles on it and never been apart

Every once in a while it would do the run on thing also
Only seemed to happen in the hotter summer time not in cooler spring or fall
I always ran the cheapest gas I could find in that car so that is what I was blaming it on but it never did it often enough to tinker with it to see why it was happening

I did notice it was always harder to start the next time after it happened as it seemed to have used up all the gas in the carb so it took more cranking to get things filled up again to start

[John_Kunkel]

Things left to try:
*Throttle blade opening.

As you probably know, the factory dealt with this problem by installing an idle-stop solenoid...completely closed the butterflies when the key was turned OFF.

An alternative for stick-shift cars is to simply put it in top gear before shutdown, then turn the key OFF and ease the clutch pedal out.

[Kern Dog]

Thanks. That feels like a "cheat" for me. I know that it does the trick but it should just shut down normally.
The idle stop solenoid idea is certainly a worthy one that I am considering.

[metallicareload99]

Sounds like you've covered most of the basics, do you have some type of insulator/spacer under the carb? Fuel pressure regulated with bypass or? The plugs seem like a good heat range to me 

What's the idle and total timing? With 800 RPM idle I would think the idle transfer slots are looking good. Four corner idle carburetor?

[Mopar Nut]

Did it run-on when you had your flat tappet cam in it before?

[Kern Dog]

Sounds like you've covered most of the basics, do you have some type of insulator/spacer under the carb? Fuel pressure regulated with bypass or? The plugs seem like a good heat range to me 

What's the idle and total timing? With 800 RPM idle I would think the idle transfer slots are looking good. Four corner idle carburetor?

There is no room for any carburetor spacer or insulator.
The fuel pump is a high volume Carter. It pushed 11 psi at idle so I added a regulator set to 3.5 psi at idle, 7 psi total. Everything over 3.5 at idle and 7 at WOT goes back to the gas tank. This car never vapor locks.
The timing curve....



Yes, it is a 4 corner idle carburetor. It idles under 14.0 AFR.

[Kern Dog]

Did it run-on when you had your flat tappet cam in it before?

Yes, it did with the Lunati. It was a lot bigger than what I have in the car now.
Oddly, I'm having trouble remembering if it was dieseling that much when I had higher compression and different pistons in it before I re-rebuilt the engine in 2022.