Now what exactly is that part were you just insert a regular old washer over that plunger looking thing. What is that doing?

 

The following is from the thread on DieselRam.

Q: I realize changing the cup washer with a flat 8mm washer will increase the rack travel, but what exactly will i accomplish by additional rack travel; (Maybe smoother/predictable throttle response?) and what would i accomplish by sliding the whole afc housing forward?

A: Increased rack travel will give you more fueling, the smooth part comes in by adjusting the star wheel and smoke screw. The purpose of these mods is if the fuel plate and afc are full forward the governor lever does not follow the fuel plate completely, the afc foot stops it. To take full advantage of this mod the 180hp and smaller pumps could replace the rack plug with one from a mack application, it's not needed but it will give it just a little bit more fuel.

 

So the washer technique won't help me since I already did the mac plug.

 

The flat washer will still give you more travel.

 

The flat washer or grinding the washer will help gain the extra rack travel of the mack rack plug. With out changing or grinding the washers the stock foot will limit your rack travel before the mack plug will.