I agree with WIChix, the tension should be loosened when ZigZag stitching. A manual can be found at
https://www.singer.com/manuals Are you using the "0" cam or a different one? Does it do the same thing if you narrow the width? How does the straight stitch work in the different needle positions? There is a Service manual for the 327 & 328 at
https://manualsoncd.com/?s=singer+328&post_type=product I think that with a zigzag machine the hook has to be able to pick up the thread whether the needle is on the left or the right. I think it can be a little off for straight stitch. I heard one time about if the bobbin thread was pulled differently from one side or the other, it was a fairly easy adjustment, but if the other side then it is something major. I guess if it isn't picking up the bobbin thread at all neither of us has to worry about that.
Janey - Neat people never make the exciting discoveries I do.
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