That is actually a vacuum operated fuel pump. The bottom hose goes to a vacuum source on the intake. This line may actually connect to a tee but ends up at the intake. The top hose, fuel supply goes to carb. How it works is that when the piston is in its down travel a vacuum is pulled which pulls down the spring loaded diaphragm allowing fuel to flow into the chamber. As the piston starts up vacuum drops and the spring loaded diaphragm pushes the fuel into the carb. To check, first make sure the hoses are correct, remove the hose from the carb and crank the engine. Fuel should spirt out of fuel line in time with engine cranking. Usually when the go bad either the spring inside breaks and fuel stops flowing or the diaphragm ruptures in which case you will get fuel out the vacuum side of the pump. Your problem sounds to be in the carb. Sometimes due to the ethanol in the fuel from sitting the carb will get clogged. Even if the jets are clear the orifices in the carb can also plug. The best way to clean is to pick up a gallon of Berryman carb dip $13-15.00 at an auto parts store or Wal-Mart. Disassemble the carb of all rubber parts and let soak a few hours or overnight then rinse under warm water and blow dry with compressed air making sure to blow through all the small orifices. If you do decide to go with a regular petcock just plug off the vacuum line. I do have both type in stock.
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