How to adjust the zigzag on Singer Stylist 457?

I recently refurbished a vintage Singer Stylist 457 and it runs great except that the zigzag feature needs some kind of internal adjustment...when I move the lever that controls the width to the widest stitch (level 5) the stitches are only about an eighth of an inch wide, and level 3 is basically a straight stitch. I suspect that there is some way to fix it but I don't know how and would really appreciate some advice. Thanks and regards, JA