Answer:
This is hard to explan but I'll try. The engine should be at normal temperture. (1) Use an optical tachometer such as Snap on # MT139 or Cummins # 3377462 to read the engine rpm (2) Adjust to low idle speed at the low idle speed screw. The srew & locknut are located at the rear of the fuel injection pump. (Looking at the pump with someone you trust slowly pushing the gas pedal. The lever at the rear of the pump will rest on a set screw when the pedal is at idle position. This is the low idle set screw). Loosen the lock nut at the bottom of the screw and adjust to factory spec. From the factory manual with an automatic 750 to 800 RPM with the park brake lock, transmission in drive & air conditioning on. (Real partner TRUST is needed here). With manual trans. 780 RPM park brake set, in neutral & air condition on. You can also just use the tachometer on the dash. Note: you need a 10mm deep socket or wrench to adjust.