I am thinking either a pin hole or other air leak in the fuel lines somewhere, or an air leak (vacuum) somewhere on the engine. Reed block not sealing, crankshaft seals, leaking cylinder base gasket, something like that. This is assuming the other carburetor tried was a good one.

Check the fuel screen in both carbs again, and both the regulator and fuel pump diaphragms. Stiff regulator or distended fuel pump diaphragms can cause this...

AV8TOR