IS F9.9/66N1 THE SAME AS F15/66M ONLY FOR THE CARB.A CUSTOMER OF MINE HAS BEEN TOLD HE CAN HAVE A 15HP OUTPUT INSTEAD OF 9.9HP BY CHANGING CARB FROM YAM PART NO.66N-14301-00 TO 66M-14301-00.GOING BY THE PARTS LISTS THEY ARE THE SAME ONLY FOR THE WEIGHT. WAHT EVER CAUSES THAT DIFFERANCE.