Why does my 2-piece Briggs Flo-Jet carburetor continue to flood?

I picked a tractor up last week that did not run, the needle valve was dirty. I cleaned the needle valve with compressed air and that allowed the engine to start and run fine for about ten minutes at a time until it starts to flood (black smoke, gas leaking, firing every other cycle). So I decided to give the needle valve another shot. I cleaned the entire unit with carburetor cleaner and compressed air, burnished the needle valve and float pivot pin with a rotary tool brush, and set the float to be parallel with the upper body when the valve is closed. It is now so bad that it floods instantly, and will backfire if it runs at all. Help!