I had all of the above with my OS91FX mounted in a GP Big Stick 60. I read a lot about this problem and talked to Hobbico Services and this is the fix that worked. The problem, as explained by a tech, manifested itself in the 91 even though the needle assy is mounted the same in the 46 and 61. The fix is to remove the needle valve assy from the back plate or mod the engine with these parts. The needle valve assy for the OS65LA is fiber and will not overheat and cause vaporization of fuel and the recommended parts to mod the engine are: needle valve assy - http://www3.towerhobbies.com/cgi-bin...&I=LXMW56&P=OW, needle stay - http://www3.towerhobbies.com/cgi-bin...?&I=LXGBG0&P=M,
cover plate - http://www3.towerhobbies.com/cgi-bin...&I=LXANA7&P=OW and last but not least a gasket set
http://www3.towerhobbies.com/cgi-bin...&I=LXCM87&P=OW

I did the above mod and my OS91FX is running fine!

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AMA 16066