The compression valve assembly has many thin washers (called shims). In valving, more washers = more damping, less washers = less damping. Thicker washers = more damping, thinner washers = less damping. Narrower washers = more damping, wider washers = less damping.
In general, I don't really see much need in buying aftermarket valving for 1989+ KX's. For the KDX's, they use an older technology base (compression) valving system that DOES need to be replaced by something like a Race Tech Gold Valve system for 1991+ KDX250 and 1993+ KDX200/KDX220R to enable a lot more tunability.
HE NEEDS KX SPRINGS, NOT KDX!!
As an example on the shims, here is a picture I took of a 1991 KDX200 shock. The top line is compression and the bottom line is rebound, but just focus on compression. The 8 on the left are all the same thickness and diameter. If I was to add a 9th one, that would make more damping (slowing / restriction). If I took one away and left 7, that would make less damping.
Inside your KX forks is a base valve with some shims. By removing a few, there will be less damping, which is more typical of a setup for woods riding. There aren't too many logs, roots and boulders on a MX track and no one is hitting a rythym section to tabletop, then a triple out in the twisty woods. One setup that is awesome for one usually is not so awesome for the other.
I will try to look around on the springs, if you are interested. Just let me know.
And if you dare to run progressive springs, you can drive over and pick up about 5 sets for free.
