Since you need to increase your top tension to the max, you need to increase your bobbin tension. If there are two screws on the outside of the bobbin case, the screw you need to turn is the larger of the two, and you need to turn it slightly clockwise. (Righty tighty, lefty loosy). Only tighten it in small increments, checking the stitches it makes at each adjustment. Once you get the bobbin tension corrected, you can adjust the top thread tension to match, and it won't need to be tightened all the way. It may even need to be loosened.