just a few times i have shut off my dart and the engine has kept turning, sputtering and surging for a good 10-15 seconds before it shuts down.
what causes this?

This effect is often called dieseling. On older engines, the carbon that builds up can remain red-hot - hot enough to ignite fuel without spark long after the engine is shut off. If your throttle plate isn't closing all the way, it allows fuel to continue being drawn into your cylinders. Check to make sure your throttle plate is closing all the way. Also, consider switching to a higher octane fuel. Higher octane fuels are more resistant to pre-ignition.

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1970 four-door Dart, 225/A-904/2.45 gears. 0-60 in twenty three minutes!

NewSlanter's answer is a good one, though switching to a higher-octane fuel is an expensive band-aid, not a solution to the problem. The effect is indeed caused by excessive combustion chamber temperature is indeed the cause. Improper ignition timing, carboned-up combustion chambers, engine running too hot (for any reason from an improper thermostat or gunked-up cooling system to a restricted exhaust system or vacuum leaks), improper spark plugs...go through this list and find the problem, and the after-run will go away. Until it does, switch off the engine before shifting out of "Drive" or "Reverse", so that there's some load on the engine when you turn off the key, and the engine will stop right away.

BG 44K or Chevron Techron Concentrate are the only two bottled gookums worth messing with to clean up the combustion chambers without removing the head. The technique is to add one or the other (my preference is for 44k) to a tank of fuel. If serious cleanup is needed, add two bottles/cans. Drive around normally until the tank's almost empty, then go out on the highway, get up to 65 or 70 mph, find an uphill section, shift into 2nd gear and nail the gas. You'll see a great cloud of brown crud fly out the tailpipe. When it stops, shift back to Drive and cruise for awhile. Repeat a few times. Then head home and replace the spark plugs. This "Italian Tuneup" sounds crude and silly, but you can get rid of a lot of combustion chamber carbon that way.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

BG? Is that short for Barry Grant?

I've found an overly rich mixture to cause diseling.

As a quick bandaid, when my car used to diesel with the 1920, you can floor it when turning the key off so the mixture goes way lean and it won't diesel any more. That, combined with Dan's tip of shutting off in drive, should stop dieseling temporarily.

I've also been told squirting a bit of water into the carb basically steam cleans the combustion chambers of any carbon. Just be careful don't go crazy and pour water down the carb throat and hydrolock the block.

I always liked my car dieseling, was funny, sounded like it was possessed.

Late ignition timing with the throttle speed screw opened up to compensate will cause dieseling too.

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Official Cookie and Mater Tormentor.

Exactly right, which is why a great many American cars of the early to mid '70s are dieseling-prone: their ignition timing was late and their throttle screws cranked from the factory, as a desperation emission control measure. In the mid '70s, we started seeing idle solenoids that retracted when the ignition was shut off, closing the throttle plate completely to try to prevent the problem (another desperation measure! All this futzing around with bionic carburetors while companies like Volvo quietly installed fuel injection and achieved cars that met emission requirements *and* started and ran correctly.)

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

It's true, they do. They're right up there on my short list of no-BS chemical suppliers, which also includes Berryman's (e.g. B12 Chemtool, B9 Chemdip) and CRC (e.g. HD silicone, red-label Brakleen).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Naw, Barry Gibb. That's why it only works if you dress up in polyester and have a great big disco-spec afro.
But DON'T do both tricks simultaneously! (flooring the gas with the car in Drive is a bad idea.)

Actually, a rich mixture doesn't directly cause dieseling. Neither does a lean mixture. But, both can cause dieseling indirectly. A rich mixture rapidly coats the combustion chamber with carbon, which retains heat and can cause dieseling. A lean mixture raises the temperature of the combustion chamber such that metal edges and points can glow red hot and cause dieseling. Flooring the gas doesn't send the mixture way lean, it sends it way rich (remember the accelerator pump), which throws too much fuel on the half-fire and can knock out light dieseling. If something's really hot in the combustion chamber, though, flooring the gas will cause more dieseling, since it gives the engine a continued air supply.

Just prior to the introduction of idle stop solenoids, some car makers cleverly added a relay and ignition switch contact such that the A/C compressor was engaged momentarily on engine shutdown, to provide drag load and prevent dieseling.
Correct on both counts. This works, but has to be done with great care. Two ways of doing it with relative safety are to get a plastic 35mm film can and use that as your water container, or disconnect a small-diameter vacuum hose (e.g. choke pull-off hose), and run a long length of hose from the carb fitting for that hose into a jar of water. You don't just drop the end of the hose into the water, you dip it in for a moment and let it take a quick slurp, then let the engine expel that, then repeat, etc. With either method, you need to have the engine running fast—2000 rpm is a good speed.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

is an idle solenoid the same as a throttle position solenoid? if so, i am almost positive it no longer functions. it is always popped out and i don't think it ever retracts. i figured this was no problem since i cut out my catalytic converter. i had thought the only purpose of it was to reduce heat to the cat during deceleration from high speed.

also, my old carbuerator had an idle enrichment diaphragm but on my new one the port is closed. what does the diaphragm do and would it help me to have it? someone had closed it on the old one by putting a bolt in the hose.

An idle stop solenoid is one kind of throttle positioner solenoid. Another kind is the "throttle kicker", which increases the curb idle speed when the A/C compressor is engaged and/or the steering wheel is held at either lock (with power steering, on some makes and models). Some throttle positioner solenoids incorporate both functions. Yours is probably at least an idle stop solenoid.
No, that function would probably not be available from the solenoid on your car.
Oof. Ya got me there; I can't recall when the idle enrichment system comes into play on one of those. All my Holley 1945 references are at the office, and I try not to keep info on that fuel-mixing garbage can in my head...it could lead to brain damage! Bolts in hoses are just as schlock as carburetor "remanufacturers" deciding they know better than the engineers who designed the thing in the first place and closing off ports that should be open.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dieseling can make break your timing chain, so ya' better watch it.

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

Myyyeah, not really. If your timing chain breaks when the engine diesels, the chain was long past due anyhow. The more likely "chain" of events is for the reverse power pulses caused by dieseling to finish shucking worn teeth off one of those '70s-up nylon-over-aluminum timing chain sprockets. T-chains scarcely every break...

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

True, most bottles of magic gook aren't worth much. But let me add another good fuel additive to your list: RedLine Fuel Injector Cleaner SI-1. www.redlineoil.com

Unlike most, it is detergent based. When I had an instantaneous MPG computer (fuel flow sensor and driveshaft magnets) in the '66 Fish, I had gotten quite used to seeing a steady 22 on the highway at 67 mph. (That was with a transplanted 1976 vintage 318, a 3spd on the column, 3.23 gears.) Then, on a trip from Chicago -> NY, I ran through one tank (the "control") at 22, then poured in an entire bottle of RedLine when filling up. The second tank averaged 22 for the first 100 miles or so. By about 1/2 tank, it was closer to 23. Was averaging 23.5 by the time I ran it almost dry. Filled up again, just gasoline, most of the tank gave 23.5, though as I pulled into my destination, my last 100 miles showed a solid 24. Admittedly, my scientific control was out of whack, by feeding it petrol from a different part of the country, but still...

I put RedLine in the '89 Mustang for about 10 years, every 8-10k miles, whenever I noticed mileage falling off gradually. Always regained about 1 mpg at each bottle. With about 1/4 million miles on original injectors, no problem, car still ran like new. (Also used RedLine oil for most of that time.) When I pulled the heads at 175k for mild porting, the entire inside of the engine was notably spotlessly clean. I was impressed.

- Erik

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Lots of early Valiants and Barracudas have crossed my path.
Also a handful of other toys for variety now and then.

I pour in Red line Fuel system cleaner si1 regularly and
have noticed on my car the run on has decreased a lot.

Probably a piece of trash in the idle circuit causing it to run lean. My D-150 used to do that.

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2