If you haven't seen the internals there's a pic of one in this link. They're basically all the same design. Some are adjustable. THe pic one isn't. The adjustment screw in yours adjusts the tension on the spring using a small plate on the end of the spring and a screw which pushes on the plate. Compress the spring, it gets stronger, the diaphram moves slower and needs more vacuum to overcome the spring tension.When the vacuum is taken away the stronger spring pushes the diaphram back quicker and the advance from the pot goes to zero.
If yours is set up weak it takes less vacuum to overcome the spring and with no vacuum the diaphram is slower going back to zero.
http://www.thesamba.com/vw/forum/vie....php?p=5920512