Hi!
The OS fs.26 is very reilable if set up right. I fly at sea level an use fuel containing just 5% nitro and 15-20% oil (rest methanol) mix my own fuel. More nitro,up to 15% is of course making the OS even more reliable at idle and improving the top speed slightly. My OS fs.26 (have several) all rew around 9800-10000 rpm with a APC 10x5 and that is what you should aim for, not 11000 or 12000.
10x5 is the prop size of choice for OS and glow plug should be no other then the OS F plug.
The carb is bullit proof as it it so simple (Just a bleed screw that adjust the idle speed) and the bleed screw position is in fact not very sensitiv at all.The OS runs perfect att all idle screw positions with or without muffler pressure.
If your engine does not idle well (doesn't have instant throttle respons) you have to look elsvere for possible faults, most of the time it's the tank and tank position that is the culprit.
The OS FS .26 should have a tank size of 100-150cc (4-6oz) No more! And the tank should be placed as near the carb/engine as possible and placed according to the"tank mounting rule" (center of tank in line with carb orrifice when the plane sits horizontal).

Check the tank for leaks , especially if you use tanks with a rubber expander (most common tanks do which is No good). A much better tank is the Tettra tank which doesn't rely on the rubber expander, instead it uses a rubber neck that seals the tank opening.
Pictures below show my Marutaka DC-3 powered by two OS FS.26 swinging my own made carbon fiber 3-blade scale 10x4,5 props (rews 9800-10000 with that prop) and performance is better or the same as with any comercial 10x5 two blade prop.