If your 3aj is similar to my 1989 U.S. model xt 600 then the plastic knob controls only the rebound damping. To increase or decrease the spring preload you need to move the two large toothed nuts (they look a bit like sprockets) that thread onto the bottom of the shock absober below the spring. You need the two suspension adjusting tools in your bike's tool kit to do this, but in a pinch you can use a flat bladed screw driver and a rock as a crude sort of hammer and chisel. First loosen the bottom nut (this is the lock nut) then turn the top nut in the opposite direction, so that the length of the spring is shortened. When the suspension has been suitably stiffened, tighten the lock nut back up against the top nut. Be careful not to go too far with the top nut. If you take it off the threads I'm guessing it isn't easy to get it back on (if it's even possible with out disassembling the shock). A few turns of the top nut should make a noticible difference.