How to set the idle adjust screw
You ask "How", but you may be wondering "What IS the setting". If you do mean "How", to set the "idle" speed, it varies depending on how many cylinders are being fed. The more power impulses (cylinders) per crank revolution and the heavier the flywheel, the slower the idle RPM is that will keep the engine running smoothly. On a lightweight single cylinder single carb engine, the minimum idle speed is around 1750 RPM. On a larger multi cylinder engine with a bigger flywheel, such as a Honda Goldwing, it can be a low as 950 RPM.
You said "idle adjust" and not "low speed mixture" so I assume you mean RPM and not idle smoothness. Older cruder slider type carbs have an adjustment at the carb end of the throttle cable with a male thread and locknut. To speed up the idle you loosen the locknut and turn the male threaded section out of the carb cap to tighten the cable and lift the carb's slider farther open to allow more fuel and air to enter and thus increase the idle speed. On more sophisticated carbs there is a side screw that either keeps the linkage from closing the airflow butterfly off or wedges the slider upwards from the full closed position. On those mechanisms turning the screw clockwise usually speeds the engine up and counter clockwise slows it down. Listening to the engine will tell you the correct direction.
Since it was not obvious to you how to adjust the idle speed, it might be that you are dealing with a California EPA mandated carburetor. California is more concerned with bugs and birds than smooth idle, so they mandate an idle that is as close to causing the engine to stall frequently as possible and then mandate that the manufacturer cover all adjustments with caps and shields to stop anyone from making the engine perform smoothly. If that's the case, you should move to Texas or risk going to EPA prison.
on Oct 03, 2018