Re: Why Shimming A Fuel Pump Changes Fuel Pressure
To answer your specific question----the fuel pressure is controlled by the pressure of the diaphragm spring. By compressing the spring to a shorter height it will return the diaphragm with slightly more force. So, the pushrod moves the rocker arm which pulls the diaphragm downward against the spring. With a shim between the pushrod and rocker arm cup the spring will operate at a more compressed height.
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