It's called "dieseling" and it's common in carb'd engines. It means that the engine continues to run after the ignition is turned off.
It can be caused by: excess carbon build up, lean/rich running, hot spots in the chamber, carb not closing completely, wrong spark plugs (too hot), wrong timing, or a hot running engine.
Run some seafoam through the vacuum line off the brake booster per the seafoam instructions. If that doesn't solve it, start digging.