Normally the screw presses on a tab or plate and holds the throttle lever of the injection pump open a varying amount. You watch the lever and if it moves(rotates) as you adjust the screw then it is the idle adjustment. A locking nut is commonly used to keep the screw from moving.
- - If the engine stalls or wants to quit when you make minor adjustments in the idle speed then there is something wrong with the injection pump. For that you will need a good diesel mechanic
to diagnose the problem.
- - However, when advancing the throttle on a diesel from idle it is not uncommon for the engine to hesitate or slow down a little before it picks up rpm's. On my Perkins
40 year old diesel I normally "goose" the throttle a couple of times to get the rpm's to increase from idle. You learn want your particular engine wants and feels/sounds like over time.