There are 2 screws atop the carb. The one that retains the carb barrel is toward the low end needle, the other one has a spring loading and that is for the idle set screw, which has no purpose on a servo driven throttle. I had a similar experience with my GS40, the throttle stop screw "dissappeared", and I also discovered my throttle barrel was moving sideways. Fortunately, I caught it before the barrel retaining screw had completely backed out, I just aligned it, tightened it down good. A spare throttle screw is available, I found one at my LHS for about $1.05. Which one are you missing?