Small Engine Troubleshooting Problems? Ask a Mechanic Now.
The shut off solenoid on the bottom of the carb is often called an "anit-backfire solenoid".
Its main function is to completely shut off the flow of gas to the engine when you shut it down, to keep it from backfiring.
It is also used to help prevent gas from siphoning into your crankcase if the float needle should fail.