Top Tension knob too loose or too tight - H Test
If your bobbin does drop an inch to an inch and a half, then you will need to tighten the tension knobs. Always provide a balance when turning these knobs, in other words if you do find that your bobbin tension is good, then turn the small knob 1 whole turn clockwise, and then do the same for the larger tension knob. Only sew one of the H's that are giving you problems, and then fine turn the tension until you have it good, then make the other knobs of the other threads knobs that are sewing poorly look the same. THen sew out a complete H test.
Another test to see if thread path is off, is pull the thread from one of the needles that are sewing good, but do not unthread it, just pull it through the presser foot and towards you, note the feel of the tension, and then match it up with one of the needles that are not sewing as well, and then turn the knobs providing that balance I was talking about until the tension on that needle feels like the needle that is sewing good.
And only pull the thread with one hand, if you try to match the feel with different hands you will not be able to tell, the mind cannot work that way. Pull the thread with one hand and get a feel for the tension, and then with the same hand pull the htread from the problem needle, and you will be able to tell when you got the tension right. If after you've tightened the knobs and you feel no difference in how the thread feels, than you may want to rethread the needle, because somewhere from the cone to the needle the thread has come off a wheel or one of the disks behind the tension knobs etc...
And only pull the thread with one hand, if you try to match the feel with different hands you will not be able to tell, the mind cannot work that way. Pull the thread with one hand and get a feel for the tension, and then with the same hand pull the htread from the problem needle, and you will be able to tell when you got the tension right. If after you've tightened the knobs and you feel no difference in how the thread feels, than you may want to rethread the needle, because somewhere from the cone to the needle the thread has come off a wheel or one of the disks behind the tension knobs etc...
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