The timing may not be set right. It sounds like it is too far advanced.
Change the hall effect sensor . it is located under the distributor cap
the idle tensioner and or the distributor.
distributor shaft
did you try cleaning your fuel injectors/ carb, that happens to alot of people
Alot of times a racing idle is caused by an airleak after the carb. or throdle body. check to see if the PCV (postive crank vent) is disconnected. Alot of times these are located, depending on madel, on the top of the valve cover near the fire wall, or under the intake just behind the distributor. It's not easy to see, but often the vac. line will crack.
you don't computer controlled engines have idle speed set by the computer
inspect the distributor gear.
retard the distributor or replace the idle air control valve and MAF sensor
With mine, the rough idle was fixed by changing the spark plug wires.
try to adjust the distributor.
The screw on the side throttle body If its not a PCM with electronic engine idle control. Most of today veichles you can not adjust idle. What motor do you have cummins? Like can people be a little more specific?? Pull the Spout Connector Before changing Idle Screw and make sure its up to operating temp then turn the idle screw on the Throttlebody.Replace the Spout Connector then all should be well,If you disconnect the Battery for any length of time you must remove the Spout Connector again to allow it to Relearn the Position.
1. Engine Idle speed set too low? 2. One or more cylinders misfiring at low idle? (Spark plugs, plug wires, distributor cap or distributor rotor would be the common culprits.)