I did that on an M20 once, it helped a bit. It's a bit risky though - I'd only try one notch at a time. The AFM is pretty important to the DME fuel curve - it's easy to fix the idle but mess up the rest of the AFM output curve to the DME though.
Be sure you mark the original position of the wheel so it can return to original position if necessary. Also be sure to seal the cover back on with a bit of sensor-safe RTV. You don't want to corrode anything in there.