The side with 4 small screws is covering the diaphragm that controls your bowl level and main jet. There is a metal disk on the inside of that diaphragm. When the bowl begins to get sucked dry, the outside air pressure pushes down on that diaphragm and it pushes on a little lever. The other end of that lever is connected to the main jet. When the lever is pushed, it raises the jet and allows more fuel to pass into the bowl. The pump (other side of the carb) is pumping and pushing against the jet all the time, but it can't push the jet open until the bowl diaphragm pushes down on that lever and raises the jet out of the seat. Fuel rushes into the bowl and fills it up. The fuel displaces the space previsouly occupied by the diaphragm. That forces the diaphragm to raise back to a neutral setting, and then the pressure on both sides is equal. That lowers the jet back into place and fuel flow into the bowl stops.

The side with one large screw is the side where the actual pump diaphragm is located. The pump diaphragm has two little tabs that cover small holes in the carb casting. These little tabs flutter up and down via the crankcase pulses and thats what pumps the fuel.

There is actually a diaphragm AND a gasket on both sides:
On the side with the pump (1 big screw in the middle) the pump diaphragm (thin one) must be placed flat against the casting of the carb FIRST. Then the gasket (spacer) is placed on top of the pump diaphragm. If you put them in backwards, the little flapper valves on the pump diaphragm can't touch the carb body and then they won't seal up on each successive crankcase pulse. No pumpy. No fuel. Engine no worky.

On the side with the bowl diaphragm, you must place the thicker gasket on the carb body first. This is nothing more than a simple spacer. It prevents the actual diaphragm from setting too close to the lever and pushing down on it constantly. This would cause the main jet to remain out of it's seat and flood the bowl continuously. Engine no worky. Put the big gasket on FIRST and then place the diaphragm on top with the metal disk INSIDE (pointing to the carb casting).

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