Using the last of the fuel sometimes results in water or dirt that was in the tank finding its way down the fuel pipe. Most times it gets to the in-line fuel filter and blocks it.
I suggest you disconnect the fuel line at the carburetor input connection, turn on the fuel, and see if you get a full flow out of the pipe. If you don't, try the same test with the filter removed (temporary bypass fuel line, or just disconnect the line upstream of the filter and repeat the flow test).
Do not replace a red (gravity feed) filter with a black (fuel pump feed) type or you won't get any fuel into the carburetor.
