I was trying to turn the idle mixture screw on my -06 this weekend but I couldn't get it to move. I haven't removed the brass plug, I just assumed that the previous owner did it since I can clearly see a slot where the screw is suppose to be.
Is there a slot in the brass plug on a S40 -06? Or is it the screw I see and it just that hard to turn around? (I didn't try too hard since I didn't want to mess up the slot.)

The pictures in the link below are from my 06 S40. The last picture shows exactly where the screw is located. If you're adjusting that screw you're in the right place.  It is a standard slotted screw and should be easy to move.  Usually just needs a 1/2 turn CCW to reduce backfiring.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1221818822