Kaw550

I had adjusted.the machine, including the feed dogs based on a manual I found online. I used a razor blade as a straight edge to make sure the height was correct. I am concerned they are a little worn. There is definitely a difference compared to the new walking foot. Last night I reinstalled the thread stand and backed the tension way off on the bobbin case and tensioner. Things seems to have improved. How can I confirm the correct needle length for the machine?

I have a singer 16-141 that be belonged to my grandmother. To get started, I am working with vinyl on a small project. I have run into a problem with the machine. I am trying to sew a fairly large stitch using 135 or 96 thread. If I go really slow, one stitch at a time it is fine. When I get up to half speed the stitches start getting smaller. With a clutch motor it's tough to maintain the low RPM's. Any ideas? I have played with the tension adjustment and the knob on top that adjust the foot pressure.