I'll add another note on this -- and explain why 95shaw says to reduce the air gap:
Since the formula for centrifugal force is a function of weight of the "shoe" (or lever in the case of a disc clutch), AND the radius of the center of mass of the shoe (or lever), and square of the rpm....
When the air gap is tightened up, the levers (or wedges, or whatever they are) that move outward from centrifugal force to squeeze the disc stack will no longer fling outward as far to close the disc stack (less air gap, less movement is required). Since the center of mass of the lever no longer goes as far "outward", it's like making the lever lighter (or removing weight from the shoe of a straight centrifugal shoe clutch).
Close the air gap, and now the clutch will typically over-slip a bit, which means that typically the spring force needs to be reduced a touch to get the stall rpm back to the desired revs. End result: less "weight" (force on the stack, in this case), and less spring... makes the clutch a bit "drag-ier" below the stall rpm, which will cause it to NOT disengage quite so quickly as the revs are coming down.
Very generally speaking, clutches with a lot of spring and weight tend to act like 95shaw's driver likes it... almost feels like the engine is freewheeling until the revs get close to stall speed, and the the clutch grabs very quickly. Clutches that are light on spring and weight tend to feel a bit "lazy", as they make contact and drag earlier, yet if adjusted correctly will stall at the same rpm.
In Al's scenario of going up a hill at Laguna Seca where the revs are dropping and may go just under clutch stall rpm, it can sometimes be beneficial to *keep* the clutch engaged, depending on the track layout. In that case, removing some spring and weight, (or tightening up the air gap and reducing spring preload on a Horstman clutch), may be a fine tuning option that is beneficial.
One of the great things about disc clutches is that there are generally more "tuning" options: springs, air gap, lever size/shape/weight can all be tweaked to subtly change how the clutch functions. On top of that, experimenting with different lining (and sometimes even the exact same lining that came out of a different batch!) can be really interesting.
PM