The purge pump, which is connected directly to the carburetor, allows the operator to remove old fuel and vapors that accumulate in the carburetor after long periods of storage which often prevent easy starting. The user activates the purge pump by depressing a flexible bulb.
Depressing the bulb directs fuel and vapor through an outlet check valve or downstream line, while releasing the bulb creates a vacuum that draws fuel and vapor from an inlet check or upstream line. Depressing and releasing the bulb up to 25 times purges the upstream line and delivers vapor-free fuel to the engine, resulting in an easier start, according to FreePatentsOnline.com.
Sometimes users confuse the purge pump for a priming pump and do not depress the bulb a sufficient number of times. This leaves fuel vapor in the upstream line, resulting in difficulty starting the engine.