I picked up a used FS 91R the other day, and it runs ok, but the idle was really low, and seemed a little boggy at full throttle. A new plug helped a little, but I wanted to tune up the carb, wondering if the previous owner maybe changed the settings. I cranked the H and L screws in, to see where they currently were at, and the H was at 3 turns out, and the L at one turn out. This seems odd, I've never seen a Stihl carb run an H screw that far out. I guess I could just tune it like I have tuned many other 2 stroke Stihl carbs in the past, but this is my first 4 mix engine, and I'm suspecting the adjustment of the carb may be different than a 2 stroke. Help?