Usually, if there are loose threads under the fabric, it is the top thread that is too loose. Check your sewing sample closely--you can even test with two different color threads to help distinguish which thread is which. Loose top thread will generally be pulled DOWN and show under the fabric. Loose bobbin thread will be pulled UP and show on the top of the fabric. The ideal tension is when both threads MEET in the middle of the fabric.
First, remove the thread from the machine. Install a brand new needle. Always Raise the Presser Foot
and thread the upper thread, making certain the thread path is correct.
Check that the bobbin thread is threaded correctly and turning the proper direction in the bobbin case.
Set the upper tension to the midway point and sew a retest. A tweaking of upper tension may be necessary to get it where it should be. Generally, upper tension should make the majority of adjustments needed for tension correction. Bobbin tension is one of the last tension adjustments to make as it is very sensitive and can go awry very quickly.
If, in fact, the bobbin tension is off, the tension skrew on the bobbin case is where the adjustment is made. Before making any bobbin tension adjustments, mark where the skrew slot is located on the bobbin case with a permanent marker. This will allow you to reverse your changes if necessary. Be CAREFUL as TINY changes make HUGE differences in tension. Make only 1/8th turn adjustment, then retest before making another adjustment.
Remember that tension testing and readjustment are a necessity when sewing. The same tension that will work with light weight thread and fine fabric will most likely not work when using heavy thread on drapery fabric. Get comfortable with making tension adjustments to get the ideal tension for each project.
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