Attitude Indicator Question

I have a vacuum operated Sigma-Tek AI. When it shows wings level the plane is turning a little and it does not agree with the turn coordinator. When the turn coordinator shows wings level, the plane is not turning. Is there an easy adjustment for this or is this a mounting error and the only fix is to mount the instrument more precisely? This is in an RV-6.

Thanks in advance for your advice,
Hank

These things sense gravity for alignment. Yes, to fix your problem, you need to slot the holes and rotate the AI.