It could also be just the opposite, too rich a mixture has caused carbon build up in the chambers, and a hot piece of carbon in a combustion chamber is the ignition source. In that case, lean it out, lower the idle a tad, and put a can of good carbon cleaner in the gas tank, such as GM's Carbon-X, or Chevron's Techron. Even though the can of "stuff" says you can pour it down the carburetor, it's better to let it burn off slowly. Even if your neighbor says to pour water down the carb, don't. Cold water (or chemicals) makes valve stems look like pretzels, and the steam washes the oil off the rings, not a good thing to do. And then there's always the possibility of causing a chuck of carbon breaking off and getting stuck where it does lots of damage.
Next fill gas tank with premium fuel, use name brand gas, not independents where you don't know what you're getting. Don't use gasohol or gasoline with high alcohol content.
If after doing the above, it's still a problem, add an idle solenoid. It's powered by ignition electrics. When you shut off the ignition, the throttle closes more, killing it. Lots of Jeepsters had them but since people didn't understand how they work they tossed them.
To adjust the solenoid, disconnect the wire to it, adjust for the slowest idle possible, but do not let the throttle plates close all the way (this prevents them from wearing the ventures.) Connect the wire; adjust the position of the solenoid or the plunger for best curb idle with the plunger extended. If you no longer have the solenoid or bracket, visit a junkyard. Lots of 60's and 70's cars used them; likely donors are GM's.
Sometimes a dieseling condition actually makes the engine run backwards a moment, pumping oil OUT of things, not good. I've seen where people walked away from their cars letting it "run on." When they got back it had melted internal parts.