I too am having a problem with my OS 1.60FX. It has a burble in the midrange that is very annoying. It sounds like it is very rich in one spot in the midrange when flying. I have done everything I can think of to get rid of it. I am looking for a smooth midrange for hovering and flying and this rich condition is right in the sweet spot for hovering and flying making transistion hesitant. I was wondering if installing a 7D carb from the 108FSR would cure this problem even if I lose my remote needle in the process. Here is my setup:

QQ Yak 72"
OS 1.60FX
Bisson Muffler for 1.60FX
Cline PCFS system installed between needle valve and carb
APC 18x6W prop
OS #8,F, and A3 plugs tried

The rpm I acheve with this prop for flight is 9000+ which is more than adaquate for this plane doing 3D just need to get the midrange sorted out. Has been a problem since I owned the engine which also has seen duty in an H9 Edge.