All the primer methods work basically the same.
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Under the primer bulb are an inlet and an outlet check valve.
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When the bulb is depressed the outlet check valve is forced open and the air and fuel in the bulb passes through the outlet check valve and into the fuel tank.
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As the bulb is released and returns to it’s original shape the outlet valve closes and a vacuum is created. The vacuum from the expanding bulb draws fuel through the inlet check valve from inside the metering chamber.
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The vacuum in the metering chamber draws in on the metering diaphragm, and lifts the inlet needle off its seat.
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The open inlet needle causes the vacuum created by the primer to draw fuel from the tank, through the pump, into the metering chamber and up to the primer bulb.
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To prevent air from entering the metering chamber during primer operation there is a one-way check valve in the idle circuit and main nozzle. Every time the primer bulb is depressed this process is repeated causing any old fuel or air that was in the carburetor to be displaced by fresh fuel from the tank. The fresh fuel in the carburetor makes the engine easier to start.
Note that once the metering chamber has been filled with fresh fuel continuing to push on the primer bulb will not help improve the starting of the engine.
handymn, May 2010
