sewing machine adjustment question

Sometimes I get a good looking row of stitches on top but on the bottom of
the fabric is an unruly mess. What causes this. BTW, I have an old (1921)
Singer with a walking foot. Fabric is vinyl.

I am trying to sew a welt into a seam. Four layers all together. When I do
this the above happens more often than not. I think it is probably tension
adjustment but I don't know which way to adjust. (tighten or loosen?). BTW
if I simply place 5-6 layers on top of each other the machine sews quite
well.

All advise appreciated.

Thanks,

Ivan Vegvary


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