Looking for the factory sitting for the tenisons

try changing the needle
adjust the top tenison one number or letter higher, using a scrap
piece of cloth to check the stitches till they look normal
clean under the needle plate and in the bobbin area for lint

it's possible that the gear or cam that make the zig zag stitch is
worn or broken or loose, there's no way to know for sure with out
having a sewing machine shop look at the machine.
you mentioned the needle, when sewing on cotton you need one
kind of needle and for knits you need a different needle, so
according to what kind of fabric that tell what kind of needle has to
be used for the stitches to come out right.
also check the top tenison--on a scrap fabric,,try to tighten the top
tension and see if the helps stop the thread loops on the underside.

If it a problem on bottom you need to work on the top of machine. Try adjusting your top tenisons. Also rethread machine.

I checked the singer website they have a free downloadable instruction manual in pdf form
Go to the home page
At the bottom of the page look for instruction manual--click on it
It will take you to another page
In the box put the model number (example 8280 ) & click search--it's the middle one

for embroidery the tension needs to be quite loose

the knob closes to your left is the upper tenison knob it should be on 4 for most things and the knob on your right is the sitich lenght knob I have always put it on 2.5 or 3 but it depends on what you are sewing

try looking at the tenison disk and see if there is any thread trapped in them

www.janomestuff.com answer it all

number 3 to 4 should be right.

The proper tension for the 537 is between 4-6. You need to make sure that your presser foot is in the up position before threading.