To adjust the idle on the 240SX, you need to mess with the IAV (Idle air valve). There are two valves like that on the throttle body, I have an excellent picture of them. I'll post it tonight, if you want.
I don't think you can do much, but replacing them sure would help, at least in the case of the FICD solenoid. Here's the S13 throttle body:
This is a view from the side, notice the position of the throttle position sensor (TPS). The Intake Air Control Valve / Auxiliary Air Control Valve is what controls the idle speed. There is also an idle speed controller on the ECU on the 89-90 (KA24E) ECU, but this will only raise the total idle speed and not fix your IACV-AAC problem. If you can't fix it, replace the IACV-AAC. A dealer can also adjust them for you.
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