Hi and welcome to Just answer.
Actually the common tension settings is between 3 and 4 if you are sewing straight and regular fabric. However, tension settings is depends on thickness of fabric. If you sewing more thicker you should adjust your tension. Please test all your tension settings.
If there is no perfect tension you need to clean your tension disc. Use scrap materials and place it between the center of your tension disc and move it forward and back. Then retread your machine with presser foot up.
Please update me for any results.
Hope this information will be helpful.
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