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Whenever the stitching is not even on the top and bottom the bobbin tension or the upper thread tension needs to be adjusted. When they are not set correctly the looser thread loops up and the stitch does not properly engage, allowing you to pull it apart.
You will need to adjust the upper tension knob; To test if the thread is being held by the tension discs properly when ready for sewing, you need to pull the thread from where it comes out of the eye of the needle-with the foot lowered. The thread on all machines should be tight enough to bend the needle when pulled. If it does not then you need to investigate why it is not tight. The most common reason is a restriction between the tension discs themselves, caused by fluff, corrosion or trapped threads.
A loose top thread leads to a bunching of thread UNDERNEATH the work (or looping on minor tension failure). Once you have done this put your numbered tension dial half way, for instance if you have a dial that goes from one to four put it on two, one to nine put in between the four and five, etc. On older machines with no tension dial numbers turn the dial clockwise until the thread bends the needle when pulled through as I have mentioned earlier. Then leave the top thread tension alone, until using a different type of thread.
Here is a link to adjusting the tension correctly.
I would verify that the upper thread path is correctly installed and that the bobbin and case is inserted correctly. I would also verify that the needle is the correct needle for the machine and that it is installed in the correct position; with the flat top section against the flat part of the holder. If the needle is bent, I would change it.
I would also verify that the dogs, that move the fabric, are in the up position.
If everything, mentioned above, looks good then you will need to have the hook/needle timing checked. The needle and hook tip must meet at the right time for the thread to be picked up. If the timing is off, adjustment is required and I would have this fixed by a professional.
I may have access to your manual, but I would need you to provide the model number, please.
If you need further help or clarification on this answer, please do not hesitate to reply to this post.
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Best of Luck, Brian
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