I have the same problem i can not get my FS-91 Surpass to idle between 2000 & 2500 R.P.M. when i leave the glow battery on the idle is OK and the transition from idle to full power is great but when i remove the battery the engine's idle goes rich then quits.
the transition from idle to full power is rich and slow. i have changed the glow plug and tried to adjust the idle mix setting but what ever i do it runs the same way.

my fuel tank is about 1/4" above the centerline of the carb. i can not make it equal to centerline of carb. because of plane design.

the props i used is Master 14x6 and a 3D 15x4 fuel used is wildcat 10% nitro 18% oil, 15% nitro 18% oil and YS 20% nitro 20% oil.

i think maybe i could try a new carb. what do you think? I just replaced the cylinder liner, piston ring and both valve assemblies
because i burned the exhaust valve i think it was the broken valve spring that did it.