Before I started messing with a idle screw on a TBI or a TPI, I would want to find a book that covers that setup. The reason I say that is because any time you change the throttle position setting, you are also changing the TPS voltage. On a lot of these the TPS is not adjustable. Most of the time the IAC is disconnected when the idle screw is adj and the timing connecter plug may even be opened. I would for sure want to know what the idle voltage should be for the TPS. As you know, it is nothing more than a variable resister like the volume control on a old radio and they do wear out. Misadjusted or worn sends the wrong voltage signal to the computer and the wrong control signal comes back to the IAC.

'67 GMC 3/4 292 4spd