There HAS to be something causing overfueling there.
One thing you could try that I do if I think I may have an overfuel problem is to stop the fuel supply to the carb while it's running and see if it smooths out as the fuel in the carb gets lower.
Any chance you can try this with your fuel system?
(Disconnect an electric pump temporarily, or pinch a rubber line maybe?)
I'll tell you what you need to know REAL fast.