Bobbin thread too loose moves fast when thread is pulled

If the thread is feeding through the bobbin too fast (is the bobbin thread showing on the top of the fabric when you stitch?), then the bobbin tension needs to be tightened. Make sure your upper tension is set at the midpoint (this is the standard factory setting).

Now, using a permanent marker, mark the bobbin case next to the screw groove (so you'll know where the original setting was and you can return it to that position if the adjustment fails). Use a small screwdriver and turn the screw 1/8 turn clockwise (tightens). (Make only 1/8 turns because a minor change will result in a big adjustment.) Retest your stitching. If the bobbin thread is still too loose, make another 1/8 adjustment until the two threads, upper thread and bobbin thread, meet in the middle of the fabric.

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***Adjust the top thread. It must be fed through the tension properly. It is best to feed the thread with the pressure foot up. While threading the machine give small tugs to check for stuck areas. Fix these even if you have to replace the thread.
***Don't thread needle yet. Now, Set the pressure foot down. Give small tugs. There should be some tugs. Adjust the tension up and down while doing this to make sure the tension is functioning.
***Thread the needle and set the tension to four. Adjust if necessary as you sew.
That is your problem. However, I've placed a guide how to set the tension of the bobbin.