No idle adjustment, the system is self regulating. The system has to work right to be right. If the car is a 86-89 there is an electrical solution that adds about 50rpms.
Does the car idle the same speed with the idle air valve unplugged? If so my guess is that its stuck. The idle valve in the unpowered position runs the car at a small bit faster than spec idling in neutral with the A/C off. Normally this would be maybe 700rpm with the electronically corrected value being 600rpm. When the unplugged system has the car placed in gear the constant amount of air is insufficient to maintain the idle under the new load and could drop to say 500rpm. The electronically corrected system will open the valve and stay at 600rpm.
Bosch Master, ASE Master, L1
33 years MB technician