V-Minion: X Graph good, Y Graph bad

I cannot for the life of me get the Y axis on this thing to tune well. The printer is sitting on a heavy duty shelf with a combination of a concrete paver and a rubber paver underneath it. I've made sure all the screws on the Y assembly are secured, and I've tried adjusting belt tension, but to no avail. The rail moves very smooth without the belts attached, so I know it's not binding. The ADXL is mounted to the center of the bed with thick VHB tape, so all the readings it's getting are from the center of mass.