hi have my 91fx on a 60 size airframe alsoso the engine runs mid range most of the flighti was on a 13/7 apcwent to a 13/6 apc to improve the mid range runit did (14/7apc did not work for me )-you must watch the fuel line before the carb on these enginesat midrange only you will see air in the line-it goes away at full boreif you have the engine with the back plate and needle valve as one casting -the design is the problemcontact OS they have a new back plate and nv for you free- peak the top end by ear and richen the nv almost 1/2 a turn on the top end (all my other engines require 1/4 turn rich here ) found that to help the mid range run a great deal i am sure you were overheating the engine on the 15/6 apc as i did also-when you reach top end RPM you must go richer to allow for the fuel drop in the tank during the flightwith this engine you must go richer than with most other engines