You set the idle RPM with your transmitter throttle trim. I think you might be looking at the barrel retainer screw, that screw holds throttle barrel in place, the idle mixture screw is inside the barrel on the right, in the middle of the throttle control arm. It does not set speed, but the mixture. Sounds like you need a manual, you can download one from here.

http://www.acehobby.com/ace/manual.htm