On the 89 L-jetrtonic spiders this is done with a couple of nuts on the air intake plenum...but we have Motronic systems and I think this is all run off the ECU. Papajam will no doubt add his decades of experience here
However you could check the idle speed actuator which might be dirty and sticking slightly, this needs a clean up once in a while (carb cleaner or the like)...easy to do. Also check the O-ring it fits into is still sealing and hasn't gone brittle on you. (You should feel the unit feeds into the air intake plenum nice and firmly, and, when removing the clip on the electric plug, it might spring off into no man's land!)
Careful of the small mounting bolt on the bracket (loosen to remove unit) and don't overtighten it, as it goes into very soft aluminium and you do not want to pull the threads out!