It really sounds like the midrange is rich and the regulator controls that! How much did you turn it in? When you do, you should richen up the low and hi needles and then retune. On past YS's I've had 1 or 2 that needed to be in almost 2 turns but 90% of the time I only turned it in 1/4 to 3/4 turn. At normal altitude (0-2000ft) and the newer YS (63/91..) only the low and hi needle usually needs to be adjusted. Also if its inverted! What size prop? Use a 12x8 for now. The 63 does have a hesitant midrange. A little more than other YS's. It may be accentuated by your altitude but you should still be able to get it to run at mid throttle.