fuel pump ?

I have a 48hp SPL Johnson motor, runs pretty good but recently started surging at WOT. In any case
I took part of the fuel pump apart the other day, I think the screen cup screw was loose so I hope that was the surge problem.
My question is (fuel pump diagram below) , what makes this thing pump fuel, I know its a diaphram type. Does it have something to do with that bottom center screw that attaches to the engine? There are 3 screws going to the engine, BTW.
Tim

If you open up the pump housing (figure 1)there will be a spring and diaphragm inside. How it pumps? I have no idea but I rebuilt the one on my 48 SPL and it was easy.

[youtube]Ljb7R09f-8k[/youtube]

As pictures are nice..

A non-mechanical pump works with the spring (if any) and diaphragm in combination with vacuum from the crank case. The vacuum from the crank case causes the above actions. which makes it a simple pump.

Very good pictures, thanks a lot guys, I guess having that screw in the vacuum port confused me, seemed like a weird place to put a screw, right where you want unrestricted vacuum flow.
Tim