Many carbruetor models use an idle solenoid to prevent “dieseling” or “after-run”.
Many things that have been done to lower emissions have enhanced the possibility of dieseling. Higher idle speeds, leaner air-fuel mixtures, retarded ignition timing, higher operating temperature, all contribute to dieseling.
When the ignition is turned on, the solenoid is energized moving the plunger outward. The idle RPM is adjusted at the solenoid. When the ignition is turned on, the solenoid is energized moving the plunger outward. The idle RPS is adjusted at the solenoid. When the ignition is turned off the solenoid is de-energized, the plunger moves inward allowing the throttle valve to close enough to virtually shut off the air supply, causing the engine to stop running immediately.