Seems to me I once worked on on a 420 that did this for the owner just after he re-built his carb. Sticks in my mind his kit had a new throttle shaft but it had teh lever on the opposite side of the plate from the original! That made it open the throttle when the governor moved to slow it down and closed the throttle when the engine went slower.
Little bugger would wind up like no tomorrow & the switch was all that saved it! If your NEW carb lever works the opposite way on the new carb from the old carb..... That would do it!