idle adjustment questions

[Louie1411]

The idle on my 85.5 na 944 is between 700 and 800, usually on the low side but it fluctuates and briefly drops even lower when the AC is on. Where is the idle adjustment screw? There is a yellow plastic cup-shaped 3/8" disc that spins freely where I thought the idle adjustment screw should be. Will turning the throttle stop screw work? Also I know I must defeat the idle stabilizer at the diagnostic according to a service bulletin I have in hand. Where on earth is the diagnostic plug? I recently installed a new O2 sensor and a fuel pressure regulator-neither of which affected the idle speed nor helped slight hestiation bug I can't seem to work out. Thanks