While you wait for answers, we may have found some information which could help you.
Click here for Singer Replace Bobbinhow to get the tension right on my machine?
Singer zigzag 457 Stylist not threading bobbin/making stitch?
Singer 507?
the thread is bunching on underside of fabric it looks like birdnest?
Engine revving up ?
Need a service manual for a Singer Treadle Sewing Machine Model 66-1?