I would guess the top is the air bleed screw. I'd also bet a dollar yours is an FP series. Go to the following site and download the 40LA manual. Should get you close. As a rule of thumb, start with the airbleed gap half open (or half closed) and, I think, two turns back out on the low-idle screw after setting the throttle to idle turning in the screw gently. I'm not sure on the initial "factory reset" for the idle - especially not knowing your specific engine.

http://www.osengines.com/manuals/index.html