Have you tried temporary mounting your trandsducer on a stick just for testing? I know that my left side is usually always darker on my boat as well, but then I usually cruise past objects and the shoreline on the drivers side. Hence shallower water on the right side, deeper water on the left. If you mount your transducer temporarily on a pole you could spin it 180 degrees quickly and see if the darker side moves with the rotation of your transducer or not. I also have to tilt my motor up to get brighter images on my left side. Let us know what you find out.