>1) On the underside, there are two thumbwheels and four plastic screw 
>adjusters.  What do they do?
>
>2) Is there a mode for using the joystick in a kind of floating mode 
>i.e. not auto centering? (Suitable for drawing graphics)

  Yes, it can.  Best I can recall is you move the joystick AWAY from the two
screws that are closest together, and while holding the joystick, turn those 2
screws 1/4 turn, and then move the joystick towards the 2 closest screws, and
then turn the other 2 screws 1/4 turn.  Or something to that effect. Play with
it for a while like that.  All the screws should fave the little "v" the same
way.  Either the notch is inline with the "v" or perpendicular.  Once you get
the joystick to be in a free mode, to return to a springy mode, just turn the
screws back to where they are all oriented the same way, by 1/4 turn.  But to
make the joystick have no spring action, you must move the joystick towards and
away from the 2 closest screws and hold it there while turning the screws.

  The thumbwheels help to find the center, you'll need to be in a program and
play with the wheels to see what they do.

Raymond