The manual says:

1. Run engine until you reach normal operating temperature.
2. Switch ignition off.
3. Switch ignition on, wait 5 seconds, then disconnect the connector to the idle speed controller.
4. Switch ignition off.
5. Wait 15 seconds then reconnect idle speed controller.
6. Repeat two more times (steps 3 – 5)
7. On last repeat, do not reconnect idle speed controller.
8. Start the engine.

Check the base idle speed, which should be at 550-600rpm.
If the idle speed is not within these limits adjust the air bypass screw
to bring the idle speed within these limits.

Switch ignition off and reconnect the idle speed controller.
Start engine and check to ensure that the idle speed is between 650-800rpm.

Where is the air bypass screw?

 

No one knows????

 

I don't know where it is.

I always ***-u-me'ed it was the screw that acts as the backstop for the throttle spring.

I don't have a picture of it right now, but if you snap the throttle back a couple of times, you'll find a screw that acts as the stop. That screw can be adjusted. I have never adjusted mine.

What's up with your idle? I'm assuming you've cleaned out the throttle body already,

 

It is the procedure for AJ6 engine, your car has an AJ16, so this isn't for your engine.
The AJ16 has no idle adjust screw and if you want to adjust the idle you have to reposition the idle control valve(stepper motor) with the diag computer at a dealer.

Is there any problem with your engine idle speed?

 

My idle's OK (could be better) but idle problems seem to be pretty common so I've been trying to figure out how the system works and come up with a reasonable troubleshooting guide. I'm getting closer but still have a way to go.

I did find this in the 1992-1996 XJS Workshop Manual - Pg. 18.1 - 10 - but I haven't had time to check it out on my car.

Under “Airflow Meter”:
“A potentiometer is incorporated in the airflow meter to provide idle fuel adjustment. This potentiometer operated completely independent of the airflow meter elements but is incorporated in the airflow meter for ease of access to its adjustment screw whack is sealed with a plastic plug.”

Under “Throttle Potentiometer”:
“The twin track throttle potentiometer is mechanically coupled to the throttle butterfly spindle and provides reference voltages for both the ECU and TCU dependent on throttle position.”

 

Ahhhh - Now I understand!
I was referencing the XJS Service Manual for the 4.0L Engine. Foolish me didn't notice the copyright was 1991 and they were referring to the AJ6.
The AJ16 manual clearly states:
"The non-adjustable fixed idle air bypass provides a base idle setting."
and
"The non-adjustable throttle position sensor provides the ECM with a throttle position input.

Thanks for setting me straight!

 

The v12 has the AAV just behind the left intake manifold for idle adjustment - not sure about the others sorry - sounds like an dealer job.