Singer 1120 is leaving large loops on the underside every few stitches. Sometimes the thread is even broken. I have checked tension, changed needles 4 times and also changed thread 3 times.

SOURCE: thread breaking and terrible looping on backside of quilt.

Ensure that all is clean and free of lint jams....now for tension troubleshooting .......

This solution is for tension problems...if you cannot form any sort of stitch, the issue is quite different, so please let me know if you need a different problem solved.....

It is quite long, but just work through each section in order.
The "knotting up" can reveal a lot. If you have loose threads on one side or the other, the tension on the opposite side will be the culprit.

QUICK SUMMARY FIRST:
Ensure sharp new needle,
Thread guides and Bobbin are Clean & Clear of lint
Set Top Tesion to 4 ....then....
Balance Bobbin to suit.

TOP THREAD TENSION:
If the looping threads are on the underside as you sew, it is the top tension. Top tension ought to be between 4 & 6 (this variation to allow for the different weights of fabric in your
projects).

IS YOUR NEEDLE SHARP ?
If you are using a needle that has seen quite a deal of work, or you suspect it may be blunt, change it for a new one !

TOP TENSION & GUIDES:
Make sure that when you thread the machine the presser foot is up so the thread goes between the discs and not to one side, top tension between 4 and 6, and that you have threaded through all the guides, including the last one, usually on the needle arm, just above the needle clamp.

It may be there is lint trapped between the discs, this will keep them slightly apart and reduce the actual tension, sometimes dramatically.

If tensions appear correct, and the thread is definitely in the channel between the discs, but still too loose and looping, try raising presser foot and remove your thread.

Now, with a 2" (50mm) wide strip piece of fabric 8 - 10" (20 - 25cm) moistened with methylated or denatured spirit, gently insert the fabric strip and clean between the discs with
a see saw / to and fro action.

In the worst cases, gentle use of a needle to pick & remove the jam may be necessary, but be very gentle and make sure the tension is set at Zero and the presser foot is raised, (to
disengage tension plates).... do not gouge or score the plates, they need a polished surface to work correctly.

BOBBIN TENSION:
Far less common, but if the loose threads are on the top, it is bobbin tension that is loose, it too may have lint in the spring and be giving a "false" tension.

I would not recommend fiddling with bobbin tension without good reason, it may end up with missing small screws and spring pieces, however, you can take the needle plate off to clean
the hook race area (where bobbin case sits)

...this is just good housekeeping, my wife does this every time she replaces the bobbin....

just take it out and clean the bobbin case and the fixed metal hook race with a small brush to remove lint. If there is a significant amount of lint, use a vacuum and small brush to get the worst.

Then wipe all this area with a cloth or cotton bud (Q tip) moistened (not soaked) with methylated spirit, especially if there appears to be fine dirty deposits....oil and lint combine to conspire against you.

If it seems likely that you ......really ....do .....actually .....need .....to adjust the bobbin case, first check there is no lint trapped in the metal spring where the thread is tensioned.

TOP LOADER:
Drop-in Bobbin case will look similar to this image with the tension screw in the middle of the metalwork....

...the other screw at one end is holding it all together, so beware....it is not a tragedy to undo the whole lot and clean it, but very gingerly and lay the bits out in sequence and orientation, or you risk tearing your hair out !

FRONT LOADER:
....this is a bobbin case from a front loading machine and works in a very similar fashion to the top loader with drop in bobbin, again, if you dismantle it, take care so you can put it all
back properly.
FINISHING UP
GETTING THE BALANCE RIGHT:
When you are certain there's no trapped lint in top tension or bobbin, set the top tension to 4 and the bobbin tension to a point where you just begin to feel resistance.

Try using good quality thread of contrasting colours so you can more easily spot the changes.

Set your zigzag to one width less than maximum (eg. 5 of 6 ...or... 4 of 5 etc) and sew a sample for a few inches and check the result.... adjust the bobbin tension screw very little at
a time, perhaps 1/16 of a turn.

You may find you are playing with this balance for some little while and if you are putting the needleplate on and off each time begin to think it cannot be correct to do this.....BUT....it is,
and eventually, you do get a "feel" for the correct tension and then it happens quite quickly.....as a user you won't be doing it very often unless there is lint built up (or are there small hands at work around the house !?!?!)

OTHER ISSUES:
If you live near the ocean as we do, salt air can play havoc with metalwork inside and out, so to help minimise this, keep a few small packets of dessicant (silica gel) in your machine
case....no case ? then make some sort of cover !

Same applies in any damp or humid environment, keep your machine dry and dust free.

Budget for a proper full service every couple of years (more often if heavily used) and if you don't use your machine for a few years, be aware that old oil will dry out and combining with
dust and form a "clag" like glue (another reason for some sort of cover, even a teatowel !)

FINALLY, A WORD ON THREAD:
If it is worth spending the time, energy and money on making something that you would like to give lasting enjoyment......use quality thread, .......it may seem to cost a little more at the
time, but the results, ease of use and added longevity will be worth the extra, and as a bonus, your tension troubles may be fewer and further between, because there is a more consistant diameter with good thread, and less compensating to be done by your tension plates and less thread breaks !

If you want any more help with this, just post back here, or, drop a line through the "Contact Us" page at www.bargainbox.com.au

SOURCE: thread breaking on Simplicity Denim Star

Hi, when your thread breaks or frays right before in goes through the eye of the needle:

1. Change your needle - after awhile, the eye can wear down and cause your thread to break or fray, eventhough it looks like the thread isn't even going through the needle. When you change your needle, make sure to install a needle that is compatible with the thread you are using (see below).
   
2. If your thread still frays or breaks, make sure you've threaded your machine correctly. Most machines have a hook or bar right above the needle that your thread need to pass thru before you put the thread thru the eye of your needle. Sometimes that spot above the needle is really hard to get to, and sometimes the thread slips out. Always a good idea to double check. If you are still having problems, the needle you are using is probably the wrong size.
   

There are lots of needles on the market, and lots of threads. You need to make sure your needle is compatible with the thread you use for each project.

The most common needles are size 80 and 90 universal needles, which are fine for most sewing projects. But if you are using a specialty thread, you need to use a needle that is designed to work with that particular thread. For example, top stitching needles have a larger eye than a universal needle, because top stitching thread is thicker than sewing thread. The same is true for metallic thread - needles made for metallic thread an eye designed to allow the thread to pass thru without fraying. Further, if you using fine thread, you need a small needle.

FYI - unfortunately, thread sizes do not correspond directly with needle sizes. Small needle numbers indicate a needle with a small eye. But the smaller the thread size, the larger the thread. For example, size 50 thread is very fine and would usually be used with a size 70 needle. Size 30 thread is fairly thick and would probably be used with a size 90 top stitch needle.

Hope this helps, and remember, you should always change your needle after about 10 hours of sewing, no matter what size needle you are using. If you have more questions, please post again. Thanks, Ginny

SOURCE: 5932 knotting every other inch

Try oiling the hook if you haven't recently.
Check you needle plate and hook for damage, they may need to be buffed or sanded smooth.
Check your upper tension discs for lint and double check your upper thread path (proper threading).
There are other things like hook timing, needle clearance or broken check spring that may be wrong but you need a technician for those things.
best of luck!

SOURCE: singer simple sewing machine will not sew

need needle threading instructions singer 3116 simple. pictures would help. thank you

SOURCE: Needle thread looping underneath, threads not interlocking at all

If the machine is now threaded, lift the presser foot.

Now, pull about 10" of thread through the needle.

The next time you thread the machine, lift the presser foot first.

When the presser foot is up, the tension control opens and allows the thread to "seat" into the control, then when the foot is down the control closes to the indicated setting.

When threading the machine with the foot is down, thread does not enter the tension control and when you start to sew, there is no tension on the thread.

No tension = loosey, goosey, loopy stitches underneath.

Because the lack of tension on the top thread, the bobbin thread cannot lock a proper stitch.