I believe I have one of the 2nd series .91 FX. My problem is keeping full throttle rpm's constant, especially when going vertical. I tried adjusting both needles as much as I could and just when I thought I had the perfect setting, it would overload or simply quit. I installed a Perry pump and that seemed to improve performance but, it's still a bit unpredictable. Sometimes it may run perfect for 2 or 3 flights just to have the same problem reappear for no reason whatsoever, same fuel, needle setting, etc. This may be OS's problem child.