Before going into changing the mixture control settings, have the CIS tested for the fuel system pressure. CIS uses varying fuel pressure to adjust for proper mixture under varying engine loads. If you're fuel pressures are off, you're going to be chasing your tail by turning the mixture adjustment screw.
The system fuel pressure test will tell you;
1. The delivery pressure from fuel pumps is correct.
2. The CIS fuel injection system pressure is correct.
3. This in turn will tell you if the fuel system pressure regulator (know as a Warm Up Regulator aka WUR) is working within system parameters.
After those checks, you can go after other areas that may be causing the problem. The Auxiliary Air Regulator (AAR) controls the amount of air going to the engine during cold startups. These devices sometimes stick open or closed, causing all sorts of problems.
In short, I found that electro-mechanical devices (fuel pumps, AAR's, WUR's) are usually the culprits in problems like you described over mechanical devices (idle mixture screws, idle adjustment screws). That is, unless someone tampered with the idle mixture screw to mask some other problem mentioned above.
