If the throttle positioner screw, a.k.a. curb idle screw, is turned in too far; it effectively removes the idle circuit from the carburetor, and the engine idles on the main circuit. If this condition is true, then one can close the idle mixture control screw OR remove it and place it in one's pocket with very little effect.
Your description "We set the idle at rough 500 RPMs".
Does this mean you used a tachometer and set the idle at 500 RPM and the idle was rough, OR you set the idle by ear at roughly 500 RPM?
Jon
