We recently had the same problem with a 3w to the point where the engine stopped after coughing in the vertical. It ended up being a rich low end keep leaning the bottom end until it wont transition then just richen it up until it will. Then check your high end. Remember the engine always draws fuel from the low end and the high only supplements this as the venturi effect gets greater. Try this anyway it may or may not be your problem but sound very similar.