**Always follow the instructions in your repair manual when doing repair or maintenance work on Outdoor Power Equipment. Manuals can be found at the manufacturers website.**
Solenoid fuel shut off valves are used to close the main jet or main fuel circuit in the carburetor when the key is turned off. The solenoid valve works by using a spring loaded plunger to press against the main fuel circuit passage and prevent fuel from entering the passage. When the key is turned on a magnetic field is generated in the solenoid and the plunger is pulled into the body of the solenoid, thus opening the main fuel circuit. By closing the main fuel circuit when the key is turned off, the fuel in the carburetor is prevented from entering the engine, and this prevents unburned fuel from entering the engine while it is shutting down, and this helps prevent engine back fires.
When the engine is stored with fuel in the carburetor for long periods, there is a risk of the old fuel causing contamination and corrosion to develop on the solenoid plunger causing it to freeze and no long move freely. When this happens the plunger is stuck in the fully extended position, and this closes off the main fuel circuit not allowing fuel to enter the engine when the key is turned on, and this prevents the engine from starting.