Is it possible that the throttle blades are hanging up just a very slight bit, not enough to be visually seen at the throttle stop position but enough to make the change in speed you are seeing? I'd measure the EXACT distance between a point on the throttle linkage and a fixed point on the carb body to confirm/elim. Your positive nothing is hanging up the throttle (choke/fast idle etc) are all off/not interfering in any way