I'd like to echo Jeff. I had to reset my MFI after I bought the car. If your idle screw is pegged, I suspect that the other adjustments are out and then someone tried to make the car idle OK by adjusting the idle screw. If your idle is pegged in the clockwise direction, then your idle is maxed out "Rich"
You might want to download the MFI documentation from Pelican's
MFI Information . I'd read this all through first.
Unless you KNOW that the rest of the MFI is set up correctly, I'd follow Bosche's "Check - Measure - Adjust" strategy to ensure that you get it right the first time.
1) Confirm that the ignition and such is in good shape. If needed replace the tune-up parts (cap, rotor, distributor points, plugs) just to ensure that they are not causing a problem which is being masked by the idle richness.
2) Make sure that the engine is running OK through-out the range since as Jeff described, the Part Load measurement has a larger affect then the idle adjustment. Does the car have an "lean surge' or does it often foul the plugs? If you have a syncro meter, you might also want to confirm that each of the stacks is pulling the same amount of air and that nothing is out of adjustment there.
3) Finally, when you make adjustments, keep a log of the number of clicks rich/lean you change the settings for both the Part Load and Idle settings. On the 2.2 this screw clicks every 1/12 of a turn while on the 2.0 MFI it clicks every 1/6 of a turn. On both the 2.2 and the 2.0, the idle screw has clicks every 1/6 of a turn. You will find that you can pretty well zero-in on the right settings by feel even if you don't have a CO meter.
Good Luck and enjoy learning about one of the really neat features of the early 911's. I love the MFI on my car!