1. There is dirt. (Push open the needle plate, take out the bobbin case, shuttle bed, bobbin, clean it with cotton cloth, and then use thick duck feathers to clean the dirt in the gap between the feeding teeth, and then run it after dripping with engine oil);
 2. The machine head is clamped, and the spring is too tight or too loose. (Adjust the tightness to make it pull out smoothly and naturally);
3. The screws on the bobbin case are too tight or too loose. (The method is very simple, adjust the screw);
4. There are two preventive methods for thread jamming. One is to keep the thickness of the upper thread and the bottom thread consistent. The second is to thread the thread in the prescribed order and not to wear it wrong.

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