The main cause for the valve NOT to switch is the tiny O-ring that allows fuel to the top of the diaphragm swelling shut (because of ethanol in modern gas). This prevents the valve from moving, and if it does move, it can stop halfway, causing the engine to stall.

To fix it, slide the O-ring onto a 1/16" drill bit in a STOPPED variable-speed drill, hold onto the O-ring gently, and slowly bring the drill up to full speed, as you slide the O-ring off.

Owner caption September 2008 · Photo 2383 of 5000