The AFC cover has a diaphragm in it. This diaphragm sees boost pressure on one side. The diaphragm is hooked to a "foot" inside the afc. This foot interferes with the gov lever, it keeps it from contacting the plate at certain times. There is also a spring inside that basically pre-loads the diaphragm, and its preload is adjusted by the Starwheel.
Essentially, its just a simple device to limit smoke and low boost fueling.
The way it functions is quite simple. At low boost (from the turbo), the diaphragm has no/little pressure acting on it. The spring (AKA the AFC spring) keeps the "foot" towards the back of the pump (towards the firewall... ). As the boost rises, the pressure on the diaphragm increases, slowly overcoming the pre-load from the spring. This causes the foot to gradually move foward (towards the radiator). As the foot moves foward, it allows more fueling until the point when the gov lever reaches the plate. After the foot has enough boost to move out of the way, the plate then comes into play.
By sliding the afc housing foward (using the slotted holes where it mounts), you are essentially changing where the afc acts. The initial fueling will be more, and the foot will move out of the way faster. This means more low boost fuel and smoke.
By using a lighter AFC spring, such as those sold by TST, you can change how fast the foot moves out of the way, without changing the initial position of the foot. This can also be accomplished by tightening or loosening the preload, AKA the Star wheel (behind the allen cover on top of the AFC).
There are many adjustments that can be made to the AFC. Loosening the spring and sliding the housing foward are just starters.
hope that helps some more Bart,
--Jeff
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