My suggestion has nothing to do with imbalance. I also think you are barking up the wrong tree with the position control rod. If you are deadset on messing with it do as the Wolfman says. Adjust per the WSM else you will be creatimg a new problem.
Dan
^^^^^^
I'm thinking Dan may be correct. Hard to tell from the picture angle provided, but the adjustment on mine (same tractor) has never been changed and it doesn't appear to be markedly different than the OP's.