I do not have a carbon monoxide (CO) tester or access to one. I can adjust CO levels  close using the car's oxygen sensor.

With the oxygen sensor disconnected from DME, connect a voltmeter. The plug is near the master cylinder at the fire wall. Connect the second voltmeter lead to a good ground. If you set the adjusting screw on the air flow meter (AFM) with the oxygen sensor reading 0.50 Volts, It should be very close. There is a lot of electrical noise coming from the oxygen sensor and the voltage will jump about a bit. Putting a large ceramic capacitor across the inputs of the voltmeter will help the noise.

There is a cover on the AFM that blocks access to the adjusting screw. This is government required to prevent tampering for smog reasons. The cover must be drilled out.

On your AFM, remove the adjusting screw and and replace the o-ring. This prevents a vacuum leak and stop the adjusting screw from turning out. Mine was in such bad shape that the CO level was unsettable. The CO reading jumped around erratically. First screw in the adjusting screw all the way counting the turns. Be gentle as to not damage the AFM when it bottoms out. Then remove the screw, replace o-ring and reinstall. Screw in until it stops and then back it out to its original position. Now adjust the CO level.

For cars without heated oxygen sensors, run the engine for several minutes at 3,000rpm to guarantee the sensor is at working temperature.

Late model 944s will have trouble with this procedure since they will disconnect the sensor's heater as well as the oxygen sensor. They share the same connector. A small bypass must be made so that the heater element remains connected while the oxygen sensor is connected to the voltmeter.

Replacing the o-ring on the throttles idle adjusting screw at this time is a good idea.