Just throwing this out there but I just got my mower running after two weeks of swapping carburetors, changing the spark plug, removing the fuel tank and changing the fuel filter to no avail. Ended up being the manually operated fuel shut off valve. I noticed the knob for the valve was binding slightly, didn't give it much thought until everything else checked good. There is a nylon plug down in the valve with a hole drilled in it. When the valve is turned to OFF, the nylon plug turns to block the fuel from the tank. When turned to ON, the plug hole lines up with the fuel hose allowing fuel to flow. The nylon plug was bound up and was almost closed off. Figured I'd toss that out there for one more thing to try for people with these odd performance issues.
Thanks