How do you adjust the air Governor on a Cummins 325 engine?

The engine is irrelevant... you either have a Bendix or Wabco-Meritor air brake system. First, you need to locate the air governor. It could be mounted to the air compressor, to the air dryer, or on the frame rail. Next, you have to identify which type you have. Non-adjustable air governors have a blue cover; adjustable ones have a black cover. Remove the black cover. To adjust it, you'll need either a 7/16" or 11mm wrench to loosen the lock nut at the bottom of the adjustment screw, and a slotted screwdriver to turn the adjustment screw. Every 1/4 turn counterclockwise should raise the cut-out pressure by about 4 psi; every 1/4 turn clockwise should lower it by about the same. Keep in mind that your cut out pressure should be between 115 - 135 psi, and your cut in pressure no more than 25 psi below that