I just stumbled across this in some old notes:
On the ECU is a slotted head screw which connects to a potentiometer and two LEDs.
Turned fully clockwise, the potentiometer will flash the fault codes on the red and green LEDS - red = 10's, green = 1's.
Fully anti-clockwise disables the vacuum advance on the timing - that position is used to check the base idle rpm.
FACW, it is also used to adjust the air:fuel mixture, using the screw on the airflow meter, by matching the flashing of the green LED - lean is faster, rich slower - to the red LED reference flash rate. It is a bit unclear but if both LEDs are off (at idle) it is in open loop so you might have to lift the rpm off idle speed to adjust the mixtures.
The ignition timing is adjusted with the slotted screw/pot in between the two extremes. Start with the screw in the middle position, adjust the ignition timing to 15 degrees by moving the distributor body, adjust the idle rpm (back) to 700+/-50 rpm by moving the slotted screw/pot - which moves the ignition timing via the ECU. Check the ignition timing on the crank again. Repeat the process until both the ignition timing and rpm are correct (the notes I have suggest at least 3X).
I would mark the starting positions of the screws so at least you can go back to where you started from if you get lost.
I have seen a few VLs with either the distributor or the cam belt a tooth out = check if you can't get the 15 degrees with the bolt somewhere near the middle of the slot in the distributor body .