Threads underneath is the top thread coming down and not being tensioned enough by the top tension dial. So turn the top tension back to 5 and test sew. Does this produce an even stitch tension top and bottom.
If this then is still giving the looping underneath, recheck that the top thread has seated into the tension discs fully, always raise the pressure foot before threading the top of the machine so the tension discs are open fully.
Also cleaning top tension discs sometimes helps, to do this, turn tension dial right down to zero, raise pressure foot, remove all top thread and "floss" between the tension discs with the selvage edge of a clean piece of cotton fabric, if you get lots of gunk on the fabric edge from the tension discs, then keep cleaning with a fresh bit of fabric. Thread, especially, black or cheap brands, will shed dye particles and this will build up in the tension device with time. You can also clean with some denatured alcohol, just wet the fabric edge and floss again. Just don't get onto any plastic bits.
Now thread up again, set tension to 5 and check that the top thread is in tension by threading down to just before the needle, lowering the pressure foot, and pulling on the thread. It should be firm. If you can't get this to happen at all, then the tension device may have broken and will need to be serviced by a technician.
Also if you don't have a manual for your singer you can download at http://www.singerco.com/accessories/instruction-manuals
, just type in your model number and click search.