if you are having trouble with the engine running on after the key is turned off check to see if you have an electric actuator (some will call it a solenoid) that you adjust for idle speed. it should have one that pushes against the throttle on the carb when the key is on and relaxes back when you shut the ignition off. that allows the throttle plates to completely close when the key is off but also allow idle speed adjustment on the plunger from the solenoid. the reason for "run on" with a hot engine, when the key is turned off, can be the throttle is still open partially. it can continue to draw air, and so air/fuel mixture, into the engine. the hot engine can then "diesel" because the hot spots in the cylinders are hot enough to ignite the fuel mix being drawn in past the open throttle plates. some guys will rip all that stuff off the engine in an attempt to get rid of the "emissions crap" and then adjust the idle speed with the screw on the carb linkage. that leaves the throttle plates always partially open. that little solenoid item should stay and be wired in to the "hot with ignition on" slot on the fuse panel. some cars with a/c also use these to increase idle speed when the a/c clutch engages so the engine doesn't die under the sudden load of turning the a/c compressor.
here is a pic of a generic one off google.
https://www.lectriclimited.com/carbu...-stop-solenoid