Try going from the empty filter back through the fuel line to the tank and make sure you have good gas flow at every spot. Definitely fuel starvation but you'll have to check everything, blow out the lines, check the tank filter too. Said you checked the gas cap vent. Since you can see the filter is empty, what about the line going on to the carburetor? And the carburetor bowl...is it empty as well? If that's the case, then the restriction is somewhere back of the filter going to the tank. There's a GREMLIN in there hiding someplace! I had an old Ford mower doing a similar stunt. Start up and run great, then in a couple of minutes start stalling out, sputter and nearly die. Barely got back to the garage with it on 2/3 choke or full choke at times. Belonged to a neighbor. I went thru that fuel system back and forth only to find that it was a piece of foreign material in the "L" shaped fuel inlet fitting to the carb! At times it would float off the inlet and fuel would flow, then it would get sucked down on the inlet and starve it. This repeated for many days until I took off the fuel inlet, removed the needle and blew it out. That's the only thing I can imagine would cause such starvation. But may not be the case since you've got no fuel in the filter. Even if the fuel inlet float needle was stuck shut, the engine wouldn't start and you'd have a full line of fuel back thru the filter to the tank. Just for experiment sake, take another gas tank, say off a push mower and run a line to the carburetor and see what it does. You can rig one up, which is what I did on the Ford and that will help narrow things down a bit. As far as the fuel pump being in the carb, no it isn't. The fuel pump would be an external, rectangle-shaped thin unit with metal cover bolted on, most likely mounted on the engine or near it. Diaphragm,spring, vacuum operated. Good Luck!
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