As the RPM of the engine increases there is less time for the fuel to burn. The dynamic timing advance in the injector compensates for this by injecting the fuel into the cylinder earlier in the cycle. This spacer provides more room for the dynamic timing advance to travel inside the pump, which allows an engine that is running at higher RPM to advance the timing further. This should make the engine more responsive and reduce exhaust gas temperature (EGTs).
Installation is easy when you have access to the side of the pump on a work bench. It’ll be quite tricky when the injector pump is installed in the engine but still possible. The cover needs to be removed, being careful not to lose the spring or washers.
Then the shim can be installed and screwed back into place.