What do I have to do to fix the tension on my machine? I am getting lots of thread loops etc on the underside...so I'm assuming it is a problem with the tension on the top.

adjust your bobbin tension

nothing to do with bobbins
A lot to do with insufficient needle thread tension

First, try a brand new needle.

Second, consider using a better quality thread. By all means, AVOID old or bargain bin threads. Coats & Clark used to be a standard, but thread quality has improved over the past few years, so you may want to check out other brands like Aurafil, Superior, Isacord, etc. Also, cotton wrapped polyester thread releases thread lint that contributes to build-up of gunk in the machine. Some sewists have noted that they experience more sewing issues when using C&C threads. However, some machines will do better with certain thread brands than others, so some experimentation may be beneficial.

Third, check the stitch on your sample again. Generally, when thread loops appear on the underside of the fabric, it is the upper thread that is the problem. If loopy threads appear on the top of the fabric, it is the bobbin thread that is the issue. You can even use a different color in the top to help distinguish which thread is the problem.

Fourth, when threading the upper thread, Always Raise the Presser Foot during the threading process so the tension disk is released and the thread can seat properly. "Bird nesting" occurs when the thread is not seated in the tension disk.
Sewing Machine Thread Bunching Up Here Why

Birds nest under the fabric big loops of thread top side looks good...

Fifth, when the upper thread appears on the underside of the fabric, the upper thread tension is too loose. If the bobbin thread appears on the top of the fabric, the tension thread is too tight (or the bobbin thread is loose). Start out with the upper tension set at the midway point and tweak it from that point. The tension is ideal when both upper and bobbin threads meet in the middle of the fabric.

Also, remember that tension is not static--a tension setting for chiffon will most likely not work on cotton or denim. Get comfortable adjusting the tension to suit the project you are sewing.

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Remove all remnants of the thread from the bobbin area.
First thing first - thread inside bobbin should be tightly wound in. Insert bobbin into bobbin case and thread through slot and under the spring. Spring has small screw which regulates the tension. Hold the lot by thread like yo-yo and jerk a little down-up. It should drop just a little (not run down) - that is usually required bottom thread tension,
if not, regulate with the screw. Set top thread tension 3 and sew. If still looping under, increase the top tension higher till the looping disappears. Also try new needle.

What thread are you using and what are you sewing? I'm assuming the machine is stitching correctly and it is just a tension issue. I would test each thread seperatly by hand and see if one thread is too tight or too loose. Thread on the bottom usually mean the top thread is too loose however it could by the bobbin is too tight. It could also indicate a slight timing issue or a burr on the hook. Is the thread shredding or getting damaged?

It seems to be a tension problem, if the loop is underside its usually a greater pull from the underside thread, whether for bobbin thread tension too tight or upper tension too slack. As I told my late mom after reading her machine manuals, unless you are a service pro, don't mess with bobbin tension (if you have to, mark the original screw position and use specific 1/4 turns or 1/8 turns to check), just make small adjustments in your tension dial and check for results. I it requires an unusally high tension there is no choice but adjusting lower tension by yourself or in the service shop

It's most likely not the machine. It's the other things, ie needle, thread, etc. combination. For sewing knit, be sure to use a ball point needle. The needle eye size is important to so the thread is neither too small nor too large for the thread weight.

FWIW, it sounds like bird nesting. --the thread is not seated completely in the tension disk. Remove the top thread from the machine and ALWAYS RAISE the presser foot when rethreading!

Also, check that the tension is appropriate for the project. The two threads should meet in the middle of the fabric. If there is top thread showing on the underside of the fabric, the top tension is too loose. Tighten it until it until the top thread pulls the bobbin thread up into the fabric.

All About Needles SCHMETZ Needles

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no, more likely your top tension is too loose causing the loops on the underside of your fabric, loops on top would be bobbin too loose. raise presser foot and rethread top thdead pulling thread into tension fully. set top tension dial to 5 and test sew. if still looping underneath try tightening top tension by turning dial up to 6. but usually top tension should only need minor adjustment tweeks. you can test top tension by lowering pressure foot to engage tension and holding thread just above needle thread guide, pulling thread downwards . there should be resistence on it to your pull. if you cant feel any then tension device may be faulty, and require service.

looping thread on the bobbin side means that the upper tension is too loose... make sure it's threaded correctly.

take it to a serviceman.