when i turn off my car it keeps sputtering

What does "on the dot" mean? These cars were not specified "for optimum drivability," they were spec'd for emissions, and many ran very retarded.

Bumping the timing as advanced as it will run without other problems will help the run-on problem

Other causes are running way too hot, carb out of adjustment, and as mentioned retarded timing. Obviously, local OD weather conditions affects the problem.