It certainly sounds like the low end is just a bit too lean especially given that it will accelerate well from 1/4 throttle on up. If it was too rich then it would bog/hesitate when accelerating. Open up the low end just a tiny bit at a time and test for long term idle vs instant acceleration. When you get the proper mix then reset your top end RPM with a tach but the max ground based RPM you'd ever want out of this engine is probably around 9600-9800 as it will, like most engines, unwind a bit in the air so proper setting of the high end will avoid the possibility of over reving in the sir at full throttle and the resultant often disastrous valve float.