At present, some new brand combine harvesters in the domestic market still use a mechanical oil pump in their fuel supply system for the low-pressure fuel circuit. These oil pumps are typically equipped with a manual priming device, which is used to manually pump fuel, purge air, and ensure proper operation of the system.
But why is a manual pump used so often without being operated? The main reason lies in the difference between the elastic force of the return spring and the diaphragm spring. When the plunger is at the lowest point of the cam's rotation, the diaphragm is in its lowest position. At this moment, the pressure rod is pulled back, and the eccentric finger cannot reach the edge of the draw bar assembly’s square hole. As a result, the diaphragm can't move up and down, meaning the pumping process isn’t completed.
Only when the plunger is at the top of the cam's rising stroke—when it is lifted and the return spring is compressed—can the diaphragm spring push the pressure rod upward. This movement causes the pressing rod to rotate, allowing the eccentric finger to press into the square hole of the draw bar assembly. This action completes the suction phase of the fuel. Once the pressure rod is released, the diaphragm spring lifts the diaphragm again, pushing the fuel forward. Therefore, the key to successful manual priming is ensuring that the hollow plunger is at the top of the cam's rising stroke, as this gives the maximum pumping stroke and best fuel delivery.
To perform manual pumping, turn the lever by hand and pay attention to the resistance you feel. If the pressure lever feels strong when pressed, and there's a sudden increase in resistance around the 1/2 to 1/3 stroke, it indicates that the cam is in the correct position. In this case, pulling the pressure lever about 300 to 400 times should fill the filter completely. However, if there’s no resistance, or if the resistance doesn’t increase during the latter part of the stroke, the cam position may not be ideal. In such cases, you can turn the key switch and let the engine idle a few times—usually four or five cycles—to help the cam find a better pumping position.
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