If you've eliminated vacuum leaks, and don't want to spend the money to replace the catalytic converter (which is probably the root of the problem), then the only way you'll get it close is to retard the timing.
To do this, you turn the distributor clockwise. A little makes a big difference. Note that you will be significantly down on power. If I remember correctly, the timing mark will move to the right as viewed with a timing light from the front of the engine. Make sure you note where the mark is now! And yes, the mark does bounce around a bit at idle.
Also, you may want to have a shop set the mixture- there's a screw on the AMM for this. It may just be off- it needs to be adjusted while running on an exhaust gas analyzer, so you'll have to ask whether the shop has that equipment.
Good luck!
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Rob Bareiss, New London CT ::: '87 244DL/M46- 220K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K, 88 244GL 145K