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Run on is pretty common on many carbureted hi-perf motors. You can solve it several ways. Turn down your idle speed as suggested by Ron. Put your car in 4th gear at idle and let out the clutch until tach indicates a lower rpm then turn off the ignition. Or, you can install a solenoid on your carb throttle linkage that can be set to allow proper idle speed when the motor is running, but as soon as you turn off the ignition it allows the idle speed setting to drop to zero and closes the throttle plates entirely.

Bill