OK, I'll try to give you a straight answer. The rack damper is a spring loaded device that presses against the "rack" inside the injection pump at idle speed. Because this rack would have a tendency to "hunt" back and forth while trying to maintain a constant idle speed (resulting in a rough idle), this device dampens the movement and smoothes out the idle. You adjust it by screwing it in or out, the further you turn it in the more damping you will get. Too much damping can result in hard starting or stalling, too little damping can result in a rough idle. You can adjust it by unscrewing it until the idle begins to get rough, then screw it back in until it smoothes out. Eventually the spring inside the damper will get weak and it will need to be replaced. The original dampers were silver in color, and the "new improved" dampers are gold in color.

I assume you know were it is located?