RE: OS 91 FS Surpass problems
I just experienced a similar problem with the same engine. Could not get satisfactory performance out of it until I determined that I had the idle screw out of whack. Once I got it reset to the proper position per the manual I was able to properly tune it.
One clue that led me to recheck the idle screw was that in order for it to run at high speed, the main jet was only opened about 3/4 of a turn. After resetting the idle screw, the main jet tuned for high speed is now about 2 turns out.
Hope this helps.