The pump is a rubber diaphragm operated by a lobe on the camshaft. The cam pushes the operating lever outward against a spring and the diaphragm causes a vacuum to be put on the inlet line which draws fuel into the pump. As the cam lobe moves away from the pump the spring pushes the diaphragm back to its original position and forces gas into the line going to the carburetor. There are one-way check valves which are supposed to keep the fuel from flowing back to the tank. A good pump will develop 10 inches of vacuum or more on the inlet side just from cranking the engine, and hold it for a few seconds after the cranking stops. If there is fuel being pushed through the outlet line, it should have a pressure of 3-5 PSI, an d also hold pressure for a while if the outlet check valve is working.
A good pump will deliver about 1 pint of fuel in 30 seconds to a container from the fuel line at the carburetor if you run the engine at idle speed with the line disconnected. Use a long piece of rubber fuel line to keep the catch container well out of the way of any backfires, etc. If the engine won't idle for 30 seconds on the fuel in the float chamber, fix the carburetor!
Jerry
