4. Idle Adjustment
If you've just installed an SR and the idle speed isn't right then you may be tempted to go for the idle adjustment. Before you do consider this : unless the engine is a real clunker then there's a pretty good chance that the idle was spot on before it was removed. So the idle problem is more than likely due to a fault with the installation. By trying to adjust the idle you will just cover up the symptoms. Check all your hoses around the inlet area first. Any air leak into the manifold will increase idle speed.
Adjustment is not as obvious as it might seem - particularly if you're used to old school carby engines. See the throttle stop screw on the throttle body? Don't touch it. The idle is controlled by the AAC valve (Auxiliary Air Control). This valve simply bleeds air around the throttle body. It's mounted on the inlet manifold and you will find an adjustment when you peer down between the runners of the manifold on the S13 - slightly different on S14/15 but easy enough to find. Standard idle speed is 825rpm when warm.
The purpose of the throttle stop screw is to prevent the throttle butterfly from sticking in the bore. Adjustment is not normally required but may be performed by backing off the stop screw until you feel the butterfly start to stick in the bore, then screw the stop screw inwards until the butterfly is totally closed but isn't sticking in the bore. Then tighten the lock nut.
If you're setting your idle from scratch then there are a few things you need to know about. A good place to start is probably how the ECU controls idle. To work out that the engine is trying to idle it monitors the TPS voltage to see if the throttle is closed. This should be around 0.5V. Once this happens the ECU sets the "Throttle Closed" flag (this flag can be monitored using any of the diagnostic packages available). If this flag isn't being set then the ECU does not know that the engine is trying to idle and it will not try to maintain target idle speed. It opens/closes the AAC valve to allow more/less air into the engine. It will also adjust ignition timing to trim idle higher/lower. At target idle you'll see 15 degrees, if idle is too high you'll see less than 15 and if idle is too low the ECU will try to increase RPM by adding timing.
Once you're happy that everything is in order you can monitor the AAC valve signal (using Datascan etc.) and see how close you've got it. With engine warm the AAC valve signal should sit around 50%. If not, adjust the idle screw on the idle control valve until it does. At 50% the ECU can adjust up/down to trim idle speed as required.