<VV> Healthy pumps?

[Doug Mackintosh]

Here is my understanding of how the fuel pump regulates flow and pressure:
 
- When the pump rod is pushed by the cam eccentric, it pushes the diaphragm up creating a vacuum which opens the inlet valve, allowing the pump to suck fuel into the chamber.
- When the cam moves away, the spring starts to push the diaphragm back down, putting the chamber under pressure equal to the spring force divided by the diaphragm area. This pressure closes the inlet valve. 
- If the carb is looking for fuel, the pressure difference between the pump chamber and the open carb needle valve will open the pump outlet valve, allowing fuel to flow until the spring has pushed the diaphragm all the way down. The pressure delivered by the pump will be the spring force divided by the diaphragm area (with some variation depending on how much the spring is compressed at the moment).
- If the carb is not looking for fuel, there will be no pressure difference across the valve, no fuel will flow, the diaphragm will not move back down. 
- If the diaphragm is still "up" when the cam eccentric comes back around, nothing will happen. The diaphragm does not get pushed up (since it is already up) and no fuel flows into the pump chamber. This is how the pump regulates FLOW when no fuel is required by the carb.
 
So the pump outlet PRESSURE is proportional to the spring pressure.
The flow rate OUT is controlled by the carb inlet needle.
The flow rate IN is controlled by whether the diaphragm has been allowed to move back down (following the outflow of fuel) to accept another hit from the eccentric.

-- Doug Mackintosh
Corsa member since 1996
Corsa/NC member since 1996, Virtual Vairs member
Corvair owner 1969-1971 and 1996-on


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