As several people have said, idle is too high or the throttle is open too far.
That is why some car companies started to put electrical anti-dieseling throttle stops on the cars.
When ignition was off, the plunger was all the way in and the throttle was totally closed.
When the ignition was turned on, the plunger went out putting the throttle at the idle postion.
So when you turned the engine off, the throttle fully closed and shut off all air getting thru the carb, so any fuel still being sucked thru the jets had no air to combine with.
