As was said, the model IV with reversed intake headers has always made it more sensitive to vibration at the carbs, and it is well known that too much vibration causes the float needles to dance on the seats and not shut off properly. So I would do several things: make absolutely sure you have excellent sync of the carbs; even though they are new, take off the float bowls and weigh the floats (Rotax has gone thru 3 or 4 iterations of float designs in the last couple of years); make sure the engine mount isolators are not old and stiff and contributing to vibration; the carb mounting rubber sockets need to be soft and able to damp vibration; of course also do what has already been said-definitely need a check valve in the return line AND a properly sized orifice (I believe it is 0.040") the small return hose is NOT a substitute for the orifice.