Idle is electronically controlled by EDS (acronyms will drive you nuts). Early 603.96s and possibly .970s have an adjustable trim to set base electronic idle. 603.971s (92-95 US 140s) don't have an adjustable trim. There is a mechanical base idle adjustment on the IP but AFAIK it does not factor into the EDS setting; i.e., EDS will set idle to 650rpm (or whatever) regardless of mechanical base idle setting. Dave can confirm.

Sixto
87 300D