The return spring (1) tends to pull the rack (2) back to give the engine maximum fuel. But as the engine speeds up the governor (not shown here) pushes the rack forward again and the rpm settles to equilibrium.
Item 3 contains a buffer spring. As the rack is pushed forward by the governor, it buts against the spring (just not visible above). This prevents the engine from stalling.
Item 4 is an excess fuel plunger. As this is pushed up, the rack is allowed to move rack back past a trigger latch and gives additional fuel for starting. As the engine speeds up and the rack moves forward
again it passes past the trigger which falls and locates the rack in its home position. Operating the plunger should only be necessary when the engine is cold.
Operation of the excess fuel trigger can be checked by operating the rack and the plunger by hand. When the plunger is pushed up the rack should be felt to move backwards significantly.
The complete housing cover (4) can only be removed from the pump top when the BA screws at 6 are taken out.
Both hex set screws in the top can be removed to facilitate lubrication, one for the latching trigger and other for the buffer plunger.