I think you are confusing the 91CD with the 90sp. The CD range do not have gearing and operate on props of the same size as other 91 four strokes. A 14 x 7 or 8 is a pretty standard prop size for a 91 FS.
With the sp series it was pointed out to me on another forum that the engine will peak just prior to the barrel being pushed fully open, and I have found this to be the case during the running in process of my 120sp. I had the same symptoms as you where I could get full engine revs with a little movement left on the barrel, then if I pushed the barrel to It's limit the revs would drop off quite considerably, and the engine would splutter a bit. I don't know why It's like this, but now I know I'll set the throttle up so the barrel doesnt get pushed to it's stop. Incidentally, I noticed the revs were peaking when the hole in the barrel was showing clean edges all around, the edge of the hole starts to just get covered if the barrel is pushed to It's limits. I guess there is just too much travel on the barrel on my SP. I feel that if I tried to set max revs with the barrel at It's stop I would be in danger of a very lean and damaging run on what I would think was not full throttle by pulling the stick back a bit but hitting that propper fully open area (If that makes sense?) I will set the servo travel so the barrel never goes past showing a clean hole.

Walts.