I figured I'd do a little write up for the functioning of this pump. I won't be adding it to my build thread unless I'm forced to use it to pump water, as I won't be needing it to pump fuel anymore, so I'm doing this post separately.

The pump is a simple diaphragm pump, with a pair of one-way valves embedded in a plastic barrier which face opposite directions.



Removing the plastic barrier reveals a chamber with three connections. One connection is isolated from the other two, and on the plastic barrier there is a gasket which interfaces with it. This is the fuel inlet line.



In red is the fuel return line that bleeds excess fuel back to the fuel tank. It has such a small orifice to add resistance to the line so that it doesn't get preferential treatment by the pump, as pumping fuel through the needle valve in the carburetor takes more effort than just pumping the fuel in a loop.


This diaphragm is made of canvas and is coated in a layer of neoprene. When the pump goes bad, it's usually a result of deterioration of the coat. Fuel will begin to sieve through the canvas and leak from the two holes at the base of the pump.


Under the diaphragm is a spring which pushes the diaphragm up and presses the fuel out of the pump. This is an important design detail. The camshaft of the engine is not responsible for forcing the fuel out of the pump, it is responsible for contracting the spring and creating vacuum. The spring itself is what forces the fuel out of the pump and into the carburetor, and this is important as it will be inclined to do this at a consistent rate and pressure. At low RPM the fuel pressure is somewhat inhibited by the engine's speed, and at higher RPM the pump is allowed to push at its normal pressure. This is why fuel pressure is roughly consistent across the entire rev range, and why you don't get spikes in fuel pressure at high RPM.


Illustrating the engine's contracture and release of the pumping spring.

 

Thanks for posting this!

I haven't seen the inner workings of these mechanical pumps before. It is a pretty simple but effective design.