I've played around with the red one b4, yeah it's for adjusting yr auto choke's idling rpm. If u want higher auto choke rpm, just tighten the screw... and I found that the Protn ignition key's sidewall is just right for this job!
Cstung,
so what's the choke opening adjuster (blue circle) used for? How does it affect the rpm and/or air/fuel mixture?
FYI the idling adjustment screw is at the other side of the carb (front passenger side), and it's that ribbed screw which u can adjust with just your fingers. Adjust lower for lower idling fuel consumption! But not too low lah, or else yr engine will die off if u let got of the clutch a bit too early.