Of course, before trying to adjust the mixture out that way I'd check a few things first:

1) Check for leaky injectors - you can pull the whole fuel rail and all the injectors out as a unit. Lay it there and turn the key on several times to build up pressure. There should be no gas dripping, oozing, or leaking from any injector.
2) Check the cold start valve - same check - take it out, turn the key on several times to build up pressure, check for leaks.
3) Check the coolant temp sensor - I don't have the specs handy but it should have certain ohm readings when cold and hot. This sensor has a large impact on the mixture setting used by the computer. Pull the plug off the computer and check it there (consult a wiring diagram for the proper pair of terminals) first. If it doesn't look right check at the sensor on the engine to determine if it is bad wiring or a bad sensor.
4) Same checks on the intake air temp sensor - but this sensor has a very minor effect on mixture.
5) Check the fuel pressure - take the hose off the cold start valve and hook up a pressure gauge. It should be 28 psi at all times, under a load or at idle. If the pressure is too high it will run rich. This is adjustable via the bolt on the top of the fuel pressure regulator. If the pressure seems to go up and down a lot - you may need a new regulator.
6) Check the throttle position switch - pull of the cover and examine the contacts. There is one set of sliding contacts and two sets that open and close when the throttle is on/off idle. The ones that close on idle trigger the special mode on the computer that uses the adjuster on the computer box. Off idle that adjuster is ignored. So if those contacts aren't closing or are dirty the computer is never entering its special 'emissions' idle mode.
7) If all that checks out then go to town with that adjuster. Of course - this will only adjust idle mixture. If your emissions test involves rollers and load testing of the engine (as they do now in St. Lous country, Missouri) then this won't help. You'll need to tweak the mixture using the fuel pressure in that case as changes to it will affect the mixture over the whole operating range.

NOTE: From what I understand Volvo was able to avoid putting air pumps and EGR junk on their engines though the following method. At idle the computer would enter a special mode which responded to the adjuster on the box. At the factory as a final step the cars would be started up and hooked to an exhaust analyzer. The knob on the box would be tweaked by hand until the emissions were in the 'sweet' spot. Then the adjustment would be sealed with a dab of paint and the car shipped out. This fine hand tuning of the idle circuit was sufficient to allow the cars to pass the 'new car' emmisions standard which tested the cars on idle. Off idle it ignored this setting and probably ran somewhat richer.