Adjust tension Need to adjust tension of thread

Double check that bobbin thread is correctly threaded through its tensioning path.
If it sews fine when sewing a straight line or zig-zag stitch you might have to loosen off the top thread tension a bit, or tighten the bobbin tension.
There is usually a small screw that controls the bobbin tensioner that can be tightened slightly to adjust bobbin thread tension. Do this little bit at a time and test after each adjustment.
Be careful when adjusting, take clear photos before touching anything so that you can put them back where they were if need be. Good luck.

Try removing the top threads completely from the machine.
ALWAYS RAISE the presser foot== this allows the threads to seat fully in the tension disk. Thread one thread completely, then thread the other thread making sure to run it through the other tension disk (if your machine has divided tension disks).
Set the tension to the midway point.
Insert fabric, lower the presser foot, and retest. Adjust the top tension as needed,. If tunneling occurs between the two top threads and adjusting the top tension doesn't fix it, you may need to adjust the bobbin tension. However, before changing bobbin tension, I would try including tear-away or tissue paper with the fabric to add body to prevent the tunneling.

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Tension is most often adjusted at the top tension dial. Only in extreme cases where the top tension does not make the needed adjustments would you need to adjust the bobbin tension. Also, bobbin tension is very sensitive and can easily throw your machine into a tizzy.

Basically, sewing machine tension is simply a tug-of-war between the top and bobbin thread. When they are equally matched, the threads will meet in the middle of the fabric. This is the optimum tension for most sewing. If the bobbin thread is coming up to the top of the fabric, the top tension is too tight--loosen it. If the top thread is going to the underside of the fabric, the top tension needs to be tightened. The higher the number on the dial, the tighter the tension.

FWIW, tension is static. It is not one of those, set it and leave it forever deals. Whenever the fabric, needle, thread are changed, there's a good chance the tension will need to be tweaked.

Understanding Thread Tension Threads

Loosen your top tension. If it is too tight, it will pull the bobbin thread to the top of the fabric. If it continues, then it may have to do with the thread weight you are using in the bobbin or the bobbin tension needs to tightened slightly. HOWEVER, adjusting the bobbin tension is usually a last attempt. It is a sensitive adjustment.

I frequently use a lighter weight thread for quilting and need to adjust the bobbin tension. So, I have a separate bobbin case that is adjusted for that purpose only. Having to adjust the bobbin tension every time one changes the thread weight is a real pain. For most general sewing, the factory set bobbin tension should be sufficient.

Understanding Thread Tension Threads

You would need to adjust the tension screw so the tension is looser than usual. However, remember to mark the setting BEFORE making any adjustments so you will know where to return the original setting.

Bobbin Tension How and Why Tension is Adjusted

Understanding Bobbin Tension

Understanding Thread Tension Threads

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needle thread tension is not adjusted up properly tighten the setting until you have resistance when you pull the thread to thread the needle

Check that the bobbin is inserted turning the proper direction in the bobbin case and that the thread is threaded correctly through the tension control on the bobbin case.

Make sure the upper thread is threaded correctly. The factory standard upper tension setting is midway between high & lower number although this may need to be tweaked for your particular machine.

If the machine is still pulling the bobbin thread too tight, you can try tightening the upper thread tension. If it still isn't resolved, you may need to loosen the bobbin tension.

You might also want to check the presser foot pressure on your machine. That generally needs adjustment when changing from thicker to thinner fabric and vice versa.

Manual: BERNINA artista 180 Support BERNINA

Bobbin Tension How and Why Tension is Adjusted

Understanding Bobbin Tension

Understanding Thread Tension Threads

How can properly adjust my tension

Sewing Lesson 10 How to Fix Tension on Your Sewing Machine

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First of all, be absolutely certain that the problems being experienced are not a result of the upper thread tension, ie threading when the presser foot is not in the up position, thread pathway is correct, a brand-new needle compatible with the thread and fabric, etc.

Before making any adjustments to the bobbin tension, be sure to mark the bobbin case where the skrew slot sets with permanent marker or a small drop of nail polish. This will allow you to reset it to original should your adjustments not be successful. And, make only minor movements of the tension adjustment, ie 1/8th of a turn, then re-test, before making another adjustment. Tiny incremental changes will make a huge difference in the outcome.

Understanding Bobbin Tension

Bobbin Tension How and Why Tension is Adjusted

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Understanding Thread Tension Threads

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Generally, tension adjustments are made using the upper thread tension. Most likely the upper tension is set too tight and is pulling the bobbin thread up. Loosen the upper tension and try again. Tension is accurate when the upper thread and bobbin thread meet in the middle of the fabric. Whenever fabric, thread & needle are changed, there may be a need to adjust the upper tension.

HINT: When threading the upper thread, make sure the presser foot is RAISED during threading to make sure the upper tension disks are open and the thread seats properly.

Bobbin tension is only adjusted when the tensions are way out of whack. Only slight turns of the bobbin case tension screw are made in graduated movements and testing occurs after each adjustment.

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According to the manual, you should not need to adjust the bobbin tension (and I located other sites which stated the bobbin tension cannot be adjusted on this machine). Apparently, all the tension adjustment is accomplished through the upper thread::
"EN Thread Tension Upper thread tension Basic thread tension setting: "4". (1) To increase the tension, turn the dial to the next number up. To reduce the tension, turn the dial to the next number down. A. Normal thread tension for straight stitch sewing. B. Thread tension too loose for straight stitch sewing. Turn dial to higher number. C. Thread tension too tight for straight stitch sewing. Turn dial to lower number. D. Normal thread tension for zig zag and decorative sewing. Correct thread tension is when a small amount of the upper thread appears on the bottom side of fabric. Lower thread tension The bobbin tension has been set correctly at the factory, so you do not need to adjust it. Please note: - Proper tension setting is important for strong seams. - There is no single tension setting appropriate for all stitch functions, thread or fabric. - A balanced tension (identical stitches both top and bottom) is usually only desirable for straight stitch construction sewing. - 90% of all sewing will be between "3" and "5". - For zig zag and decorative sewing stitch functions, thread tension should generally be less than for straight stitch sewing. - For all decorative sewing you will always obtain a nicer stitch and less fabric puckering when the upper thread appears on the bottom side of your fabric. 1 A B C 22 D"

Doing further research, I found this statement on a machine review site:
"After reading reviews online from where I've bought my drop-in bobbin machines, I think many of the negative reviews are due to the bobbin thread coming up without laying across the bobbin. It can cause the stitches to look very sloppy and no amount of tension adjusting can fix the stitches."

There are several machine review sites wherein 4423 owners stated their disappointment in the 4423's performance. Some said the machine failed almost immediately and others said the machines developed problems when sewing heavy fabric.