Tension dial not correcting tension problems

The manual for the Bernina Bernette 65 are available from the manufacturer here: https://www.bernina.com/getdoc/6d2754b8-af09-4edb-a6a6-7a4c57965b8d/bernette-55-65-80e-90e . The three dials, from left to right, are thread tension, stitch length, and stitch width (page 9, page 12 of the PDF). See page 24 (page 42 of th ePDF for the pictures of the appearance of the correct tension versus the incorrect top thread tension. For best tension testing, use different color top and bobbin threads. Do use the same thread type and needle appropriate for your fabric and use scrap from that same fabric. In general, if the top thread is visible on the under side of the stitched fabric, increase the tension (turn the dial to a higher number). If the bobbin thread is visible on the top of the fabric, reduce the tension (lower number). Make sure that the machine is properly threaded. (Make sure that the presser foot was up during threading and is raised when you adjust the tension dial.) Start with the tension at 4 and change the tension in small steps. (Do not turn the dial more by 0.5 before stitching a line to test the tension again. You want to avoid over-correcting.) When the tension is correct, the top and bobbin threads will meet between the two layers of fabric for a normal seam.

I hope this helps.

Cindy Wells
(These steps are the same for any sewing machine with user tension adjustments.)

Is that a Bernina? Which model?

Usually, if you are experiencing improper stitching and/or broken threads, it's most often a mis-threading problem. Proper threading is EXTREMELY important to successful serging.

Remove all the thread from the machine. Use colored threads that match the machine diagrams.

ALWAYS RAISE the presser foot when threading!

Thread each thread in the ORDER specified. Usually, Upper looper first, then Lower looper, then needles.

When threading each, grasp the thread below the thread tree and below the tension dial, and give it a tug to seat the thread completely. Sometimes it will pop when it seats. If the stitching has loose looping threads, most likely the thread is not seated completely.

Verify the thread path is correct for each thread before going on to the next thread.

The best tension setting is the midway point--usually marked on the dial. Then the tensions are tweaked following testing. The colored threads will help you determine which thread is misbehaving.

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Be sure to ALWAYS RAISE the presser foot when threading the machine. Also, when threading, give each thread an extra little tug when going through the tension mechanism. (Grab the thread above and below the tension device and gently tug.) Sometimes (on my machine anyway) the thread will make a little "pop" when it seats in the tension.

Is it the bobbin tension that is very loose? Try tightening it. Also check that the bobbin has been installed in the bobbin case, turning the correct direction and that the bobbin tension is threaded properly.

Bobbin Tension How and Why Tension is Adjusted

Understanding Bobbin Tension

Understanding Thread Tension Threads

If it is the upper tension that is loose and creating "thread nests" under the fabric, the machine is either misthreaded or possibly the thread is not seated completely in the tension disk.

Remove the upper thread from the machine.
ALWAYS RAISE the presser foot and rethread the upper thread from the beginning--verify the thread path is correct.
Set the upper tension to the midway point between high & low number (the factory standard--although it may need tweaking later for your particular machine).

Retest your machine. Hopefully, the issue has been resolved. If not, it may need service.

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Fit a new needle, clean and oil the hook race, whether-or-not your manual says it doesn't need it and check that you have some tension on the thread coming out of the bobbin case and top tension unit. Are you threading the top thread with the foot up or down ? You should always thread the machine with the foot up to ensure the thread goes through the tension unit properly.

Try re-threading the machine and make sure the presser foot is up first as this opens the tension and allows the thread proper entry.

Re-thread again and make sure the presser foot is up.

you may need a new tension spring in the dial

The problem is that the nylon thread will actually cut into the race cover and create groove, it should be smooth. The only way to correct this is to replace this cover. I can not stress enough do NOT use nylon thread on your sewing machines, it will do permanent damage and cost you 100s of dollars in repairs.
I have 25 years experience working on these machines and have seen this happen time after time.

There is also a tension adjustment on teh bobbin. It's just a little screw. Turn it about 1/4 to 1/2 turn and try again. Adjust as needed a little at a time.