A mechanical fuel pump is mechanically actuated by a rocker arm or push rod without electrical assistance. Figure 4-9 shows a typical mechanical fuel pump.
The rocker arm spring holds the rocker arm in constant contact with the camshaft or eccentric.
As the end of the rocker arm moves upward, the other end of the arm pulls the fuel diaphragm downward.
The vacuum action of the diaphragm enlarges the fuel chamber drawing fuel from the fuel tank through the inlet valve and into the fuel chamber. The return stroke starting at the high point of the cam releases the compressed diaphragm spring, expelling fuel through the outlet valve.
When the immediate fuel needs of the engine are satisfied, pressure builds in the fuel line and pump chamber. This pressure forces the diaphragm/piston to make shorter and shorter strokes, until more fuel is needed in the engine.
