Autocontrol IIIB

I have the original Autocontrol IIIB in my 79 Archer. Lately it has been
flying one dot to the right of course when in GPS/NAV mode. I've set the CDI
scale to 1.0nm full scale on the GPS so the error is only about 0.2 nm. If I
set the heading but to the right a little, we fly right down the line. This
says to me that the autopilot is putting out a small bias to the left that the
GPS/NAV has to constantly correct. Is there a simple adjustment in the
autopilot to correct this? Maybe I should just vent to the newsgroup and leave
good enough alone!

Chuck