Use two layers of shirt or quilt weight cotton. Choose a sew-all weight thread, I prefer Gütermann sew-all polyester, it has very even dependable quality, but a good mercerised cotton will do too.
Do a drop test on the bobbin case, leave it dangling in the air, yank the thread a bit and it should drop a few inches. You can make a bag of beans or rice and tie it to the end of the thread, it should weigh 1 oz. It's much the same drop approach. There are tension gauges for this too if you like more accurate measuring gadgets. You adjust the screw until you get the right drop.
Then thread the machine, make sure the upper tension have gradual tension as you screw the nut in. When you pull the thread backwards there should be loose grip in an outer position, quite firm in inner position. At about a medium position you should get even stitches, what ever you do it involves a bit of test sewing and gradual adjustment to get it right.
Make sure the tenion spring in the bobbin case is clean, the same for the the parts in the top tensioner too. The check spring on the upper tension has to be in good condition.
Last edited by Mickey2; 10-17-2018 at 04:40 PM.