[Excaliber]

The symptoms suggest a stuck needle valve or bad float, more than a blown power valve. All though, with all the backfiring you could have blown PV now! I'd tear it down, get a kit and rebuild it at this point. "Rebuild" on a carb is often nothing more than a good cleaning, you may not even need a carb rebuild kit if the gaskets are good. Check the floats for binding or being 'heavy', check or replace the PV, blow out all the holes you can with compressed air/carb cleaner, adjust and go.