The reason you shouldn't adjust the mixture adjustment is that normally you won't have any way of seeing the response. Donald is using a dwell meter. I would suggest a multimeter that reads duty-cycle but the two are similar functions the dwell meter is just marked wrong.
The problem with making this adjustment without the meter is that it DOESN'T change the mixture and will make NO difference in the idle if everything is working correctly. With this adjustment you are changing the base mixture. With a properly warmed lambda system the mixture will always return to lambda (about .5-.8% CO or 14.7 to 1 airfuel ratio). If you adjust richer the system will lean it back to lambda. If you adjust leaner the system will enrichen. A properly adjusted system runs at a duty-cycle of 50/50. In this position it will have equal ability to correct lean or rich. If the system is way rich (say 90/10)it will still be at stoiciometry (lambda) mixture because it is almost out of bounds but not quite. At 90/10 there is just enough range to correct to .5-.8% CO (before cat - real mixture). If you then take this system and adjust further rich you WILL change the mixture. Once the system is overwhelmed small adjustments make BIG changes in the mixture and can not be compensated anymore. This of course will set off the check engine light as the control system is now non-functional.
Basically the point I am trying to make is that you will make NO difference in the running by adjusting this screw unless you overwhelm the system and defeat it. All slight mixture corrections are almost instantly compensated for and the mixture remains the same.
In one post recently (maybe this one) I stated that one should remove the aircleaner and slightly depress the plate on the airflow meter. If one does this an instantaneous result of mixture change results. Almost instantly it goes away as the system compensates. In this brief instant if the engine smooths out one can infer that the roughness is mixture related. This does not mean the system is running at the wrong mixture, it means that one or more cylinders are not rich enough under these uniform conditions. Possible reasons are restricted injectors or vacuum leaks.
Bosch Master, ASE Master, L1
33 years MB technician