Someone else asked about emulsion tube removal today .. thread is called "pa50 carb questions"
I know you already answered that the choke is open.. but this sounds almost identical to a closed choke. The bike runs like crap up about 8 or 10 MPH or so.. If you disassembled the carb, check it for correct assembly. And as someone else noted (frankman) that float bowl can be installed backwards and will then cause all sorts of problems.
The drain screw on the bowl should be on the right side of the bike. This screw can be accessed through a special hole in the pedal sprocket and then through a hole in the swingarm when they are lined up..
While messing with the muffler, you might think about removing it so you can check the exhaust port for carbon build-up..