Thread loops noted on underside of sewn fabric. Attempted adjusting upper tension without success. Adjusted bobbin tension screw ever so slightly to tighten tension. Rethreaded machine and bobbin according to instructions. No luck. What else could I do to correct situation. Using 100% polyester thread with on lightweight cotton.

Hi, here are a few reasons it could be.1. use same thread in bobbin and needle.2. thread too old. 3. Cheaper thread frays and knots.
4.Thread and/or needle,incorrect size for the material sewn
5. Incorrect tension for the material sewn. And 6, thread tangled underneath feed dogs or bobbin. Hope this helps. :-)

First, be sure you are using a brand-new needle and that the needle, thread, and fabric are compatible.
All About Needles

When you thread the upper thread, be sure the presser foot is ALWAYS RAISED so the thread will seat properly in the tension disk. Also, confirm that the machine thread pathway is correct.

Remember that when fabric types/weights are changed, the needle/thread should be changed accordingly. (Needles should be changed often to attain the best sewing experience and minimize unnecessary frustration.) The tension may also need to adjusted. Tension is not static. It should be adjusted to fit the project being sewn. Unless one continuously sews with the same fabric, thread, and needle type would the tension adjustment remain at the same setting, but machines do change over time so a tension setting that worked last year may not look the same this year.

The factory standard tension setting is the midway point between the high & low numbers. However, please understand that machines vary and that sometimes this standard setting may change with time and use. The midway point should only be the beginning point and is most likely suitable for medium-weight fabric. Also, usually the bobbin tension does not require frequent adjustments unless a particularly heavy- or light-weight thread is used in the bobbin--sometimes the bobbin tension will get out of whack requiring adjustment (consult the owner's manual or search the internet for instructions for adjusting bobbin tension). In the majority of cases, the tension adjustment is made to the top thread.

Test your straight stitch on your fabric (may be helpful to use a different color thread in the bobbin so it is easier to see any tension issues). If the bobbin thread is popping up to the top of the fabric, loosen the upper thread tension. If the upper thread is looping or showing on the bottom of the fabric, tighten the upper tension. Keep adjusting until the upper and bobbin threads meet in the middle of the fabric.

FWIW, the same process would be used for zig-zag stitching. Just make sure that the tension is not too tight or it will cause the fabric to pucker (or tunnel).

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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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Be very careful adjusting the tension on the bobbin case. Generally, most tension issues are regulated using the upper tension adjustment unless the bobbin case is broken or someone has attempted to adjust it incorrectly. In any event, adjustments to the bobbin case should be made ONLY 1/8 of a turn at a time--then do a test stitch. Before making any changes, mark with a permanent marker where the screwdriver slot meets the bobbin case in the event you have to return it to the original position.

If the top thread is looping or knotting under the fabric (bird nesting), the upper thread is too loose or it is threaded incorrectly.

If the top thread is pulling the bobbin thread to the top of the fabric, the upper tension is too tight.

Tension is ideal when both threads meeting in the middle of the fabric.

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Be sure to RAISE the presser foot BEFORE threading the upper thread. Set the upper tension to the mid-point which is generally the factory setting. Test. If thread is looping or bunching under the fabric, the upper tension needs to be tightened. If there is a problem above the fabric, try loosening the upper tension.

NOTE--sometimes the stitching problem is a result of the type of fabric being sewn. Most machines have a presser foot pressure setting that can be tightened or loosened to accommodate the fabric.

The tension is correct when the two threads meet in the middle of your fabric.

Is it really the bobbin thread that is bunching under the fabric, or is it really the top thread that is creating loops and "bird nests" under the fabric???

Rethread your upper thread. BEFORE you thread the upper thread, be sure to RAISE the presser foot. This releases the upper thread tension disks so the thread is firmly seated in the tension disk.

If it continues to be a problem, then the upper tension needs to be tightened. Adjust slightly, then test. Repeat until the upper and lower threads meet in the middle of the fabric.

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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.

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

Loops appear on the surface (top side) of the fabric?
Reduce the tension by turning the upper tension control dial to a lower number. If the loops still appear on the surface of the fabric after you adjust the upper tension, the bobbin thread may be too loose. In this case, turn the screw clockwise, being careful not to over-tighten the screw, approximately 30-45 degrees to increase bobbin tension. After you adjust the bobbin thread, adjust the upper thread to match the bobbin thread.
Make sure you reset before your next project.

From your description it appears that you have tried tension settings on the upper tension control adjustment knob without success.
Check that the bobin tension is adjusted correctly by taking out the bobin complete with the spool.
Pull the cotton thread and it should be a smooth light pressure if the tension is correct. If it is tight to pull or pulls freely then adjust the tension screw which is found on the outside of the bobbin. The screw is very small and requires a minature size screwdriver. The adjustment will only be a slight turn of the screw one way or the other to obtain the correct tension(depending if it was loose or tight).
Once this is set reinstall the bobin and tread up the machine, hopefully with the upper tension set at approx."2" the machine will sew correctly.