May have a pinhole leak in the fuel line, causing it to pull air
collapsed fuel line
First check to insure you have fuel flowing out of the tank and through the line. Remove the inlet side (sucking) and pressurize the tank. If fuel doesn't flow out of the hose, then you have a plugged line or the sock in the tank is full of crud.
With the inlet side (sucking) still off, crank the engine. You should get vacuum at the nipple. If not, then you have a valve issue, pump arm not engaged, or diaphragm is bad.
If you have vacuum, then hook up the inlet line, take the outlet line off, and crank it again. If you get fuel out of the pump, then the line to the carb is blocked, fuel filter plugged, or the carb float is stuck shut.
If it has a glass bowl, insure it is sealed
I know it sounds stupid, but don't get the vacuum booster (if equipped) for the wipers confused with the fuel pump.