Usually thread bunching is caused by the thread coming out of the take up lever.....rethread, use a 90/14 topstitch needle.....if you take off the walking foot, put a quilt sandwich in the machine, sew straight stitches....and your tension is ok...Don't mess with your tension. Always check your thread weight to your needle size..normally 90/14 topstitch, lengthen your stitch slightly, pull up your bobbin thread to the top before you start to stitch. Let us know how things work out...