The + and - knob is your stretch balance. If you're doing an overlock or stretch stitch, and it's not touching, you can kind of accordian the stitch with that knob. Do a forward/backward stitch, and turn the knob one way and see what it does. Then turn it the other way and see what it does. for most stitches, it will stay right in the middle.
The short stitch length could just be something gummed-up. I did have an old White with a broken part on the inside. That machine not only had no control of the stitch length, but there was no back-stitch either. So, if you've got a back-stitch, you're probably just gummed-up.