RE: OS 91 FS Surpass problems
An overly rich idle mixture will be masked somewhat if keeping the glow driver clipped on all the time.
Try this: I think it was Duke Fox that said the better way to set a carb is to leave the glow driver on, run to WOT and peak the main needle. Remove the glow driver. If the rpm drops, add more nitro. Once you get the top end nailed down 100%, then you can get the idle figured out. It's no surprise that guys running radial 4-strokes use copious amounts of nitro - it keeps the plug(s) lit better.
I also suggest using a different glow plug. I didn't care much for the OS F plug in my 4-stroke. I use Enya 3's or Fox idle bar plugs. Both work great and are half the cost of the OS F.