I'm not a mechanic, so bear with me. I just had my Holley 650 double pumper professionally rebuilt, and then dyno tuned for A/F and timing. When the car is sufficiently warm, my idle speed creeps up a couple hundred RPM and idles too high. At this warm state, if I turn the idle speed screw to the right, the idle speed increases, but to the left of the original point, there is no effect. I can't get it to idle lower at this adjustment. I don't want to mess with the A/F ratio or timing. Why does it creep up on me? Where else can I check?