Have you tried using a blind gasket under one carb at a time, and tuning the engine to run right on a single carb? Once it's running well on that one, swap the gasket, and tune the other one for the best performance. If it still gives trouble after both carbs have been tuned for the best performance individually, and you go back to a properly synchronized pair, it should be obvious that you've got one too many carburetors.
Jerry
