I would guess that there is some kind of obstruction, probably a small chunk of dirt of some kind blocking the main needle valve. Try hooking up a clean piece of fuel tubing to the connection at the main jet, and blow through it, you should hear some air going through with the carb at full throttle. If that works, try blowing air backwards through the carb pickup line. Is there the possibility that the pickup and vent lines (from the muffler) got swapped? It definitely won't prime if you're sucking air...