I have a 91FX on a Goldberg Ultimate that runs erratic in the midrange around half throttle.
The engine is well broke in with about 25 tankfulls of fuel, the engine is mounted so that the cylinder is at an 8 O'clock position and I have to run the Idle mixture a bit lean, about 5 turns out.
The main needle valve is at 1 3/4 turns out. At full throttle I tune it for max rpm then back off about 4 clicks, at this setting you can back it down to half throttle and hear it missing, it sounds better if I back off the needle another 1/4 turn.

At these settings, (5 turns on idle and 1 3/4 on high needle)
Idle and low speed is fine, great transition to full throttle, smooth running and great power at full throttle. Has good vertical and dont miss it being tuned to max rpm. It just sounds lousy at the midrange. Using stock muffler, have tried different fuels, using os 8 plug and 13x8 APC. I can back out the high speed needle up to another half turn and it still makes no difference in the midrange.