What year 9.8 is it? Mine (1974) only has one adjustment. It is not an rpm screw, it is the low speed idle mixture screw. The manual calls for a starting point of 1.5 turns out from fully seated. To fully seat it, gently screw it in all the way until it gently seats. Note I used the word gently twice. You don't want to crank on it and damage the needle valve.
Then, once you get it started, let it warm up. Then turn the screw out in increments of 1/8 turn waiting several seconds between adjustment. When it begins to run rougher, stop that adjustment. Then begin to turn it in 1/8 turn at a time, again waiting several seconds between each 1/8 turn. It will get better, and then will get to the point where it gets worse again. Remember how many turns (to the nearest 1/8) that you went from the furthest out to the furthest in. Your sweet spot should be 1/2 way between the two - fine tune from there, again waiting a few seconds between adjustments. Your goal is to have the idle rpms as slow as you can while still running smoothly and not killing.
Oh, by the way, this is best done in the water (or barrel) in gear - i.e. under load. I use a tub, but use reverse gear instead of forward because it doesn't kick as much water out of the tub.
