The thread on the bottom of huge piece I am sewing is very loose. I have tried unsuccessfully to adjust? Any suggestions?

Perhaps there is lent, thread pieces, or gunk in the thread tensions. Remove all the thread. Loosen the tension disks all the way, raise the presser foot, gently pull a piece of pearl cotton dampened with rubbing alcohol Down through the thread tensions a couple of times.

Make sure the presser foot is raised. Rethread the machine, set the tensions to the mid-range and test. If it continues to be a problem, the tension disks could be worn.

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there are two thread tensioners on a machine
the first one is on the front of the machine for the top thread
the second is the small screw that holds the small flat spring under which you pull the thread after you put the bobbin in the holder
as you put the bobbin in the holder you have to pull the thread around the side until it slips into a small slot then under a retainer spring
that spring is adjustable by that small screw
the amount of tension added or released by that screw is minute so as little as 1/8 turn has a big bearing on the amount of tension of the bobbin thread so turn it just a small bit at a time until the thread tension is correct

turn tension adjuster to 0 put some thread between the discs and pull it backwards and forwards, gradually turn the adjuster to establish whether the tensioner is working properly. at #0 slight resistance should be felt and at #9 it should be hard to pull the thread between the discs

try adjusting the top thread tension just a little and then sew on a scrap piece of fabric till the bottom stitch look correct.

try adjusting the top thread tension one number or letter higher
sewing on a scrap piece of fabric till the stitch looks normal.

try adjusting the top thread tension one number or letter higher
and sew on a scrap piece of fabric till the stitch looks normal

If you have the instruction manual, you might read the chapter which describes the proper adjustment for threads and fabrics.
If you do not have one, try to find it on eBay. It is the huge source of everything.

Firstly you can download a manual from the Singer website, link is
http://www.singerco.com/accessories/instruction-manuals, type in "W750" and click search.

Once youve got the manual downloaded, I'd suggest you print it out and get it bound in covers so it can stay with the machine.

Then trouble shoot the tension, this sounds to me like you've got insufficient or no tension on either top, bottom or both threads. You need to revise the threading up and make sure you always thread up the top with the pressure foot raised so the thread properly seats into the tension discs. Set top tension on the normal mark between 3 and 4 and make sure when you load the bobbin into the bobbin holder, that the thread gets clicked into the tension spring completely.

Now test sew again, and tighten up the top tension a fraction if it shows on the underside. You can't adjust the bobbin tension on these machines or at least the manual doesn't show any adjustment, just says to adjust the top thread tension.

As a rule of thumb, change your sewing machine's needle for each new sewing project too as this will help with stitching quality.

If you can't get it looking any better consider taking the machine back to the shop (if its fairly new) and getting it checked.

If you are planning to sew lots of things, you may want to consider buying a higher level quality machine than this one, it is an entry level machine and there is huge variation in stitching quality between the cheaper models and the higher end quality ones. Just depends on your sewing needs and desires I guess as to whether its worth investing more $$.

try changing the needle
then check the top thread tension, usually 3 or 4 is good, check by sewing on a scrap piece of fabric
clean under the needle plate for loose threads or lint & also in the bobbin area for the same
make sure the bobbin is inserted correctly

Actually, it sounds like your upper tension is too loose. If the upper tension is too loose, it won't pull the bobbin thread up into the fabric properly. Check out my suggestions on Adjusting Tension and see if that helps.

Robbie