If the fuel pump isn't sucking, chances are the crank seals are leaking. When I got my marathon, it wouldn't suck any fuel at all on its own. I could shoot some gas in the spark plug hole and it would run, but fuel pump did nothing. And the guy I got it from had just replaced the fuel pump with a brand new one with no change, similar to you.
My crank seal behind the flywheel was completely blown out of its seat and dangling around on the crankshaft. Put a new crank seal in it and I put a fuel filter between the pump and the carb, as I always do. It INSTANTLY filled the filter with gas and fired up. Made some noise I thought was the clutch, but ran great. for about 10 minutes then I found out that noise wasn't the clutch
the rest, as they say, was history.
I'd put money on leaking crank seals, not allowing the motor to pull enough vacuum for the pump to work.