When you have both carbs off look at all four sets of butterflys on the throttle shafts, use a feeler gauge to see if there opened the same(front and rear on both carbs.) on the idle circuits, if not adjust them so they are BTW, di dyou buy those carbs new or used? If used look at the idle and high speeds bleeds as well as the idel fuel restrictors in the metering blocks The RPM drop sounds good but it may be loading up aftet idling a small amount of time in gear