You need to tighten the bottom and top stitches both. First, tighten the bobbin slightly. Now, tighten the top stitches the same amount of pressure as the bobbin. The attempt is to have both the bottom and top stitches exactly the same so the final stitch ends up in the center of the fabric. Observe the stitch and readjust if necessary.
If the top thread shows on the underside of the fabric, the upper tension needs to be tightened. The object is to have an equal tug of war between the two threads. It is equal when the top and bobbin threads meet in the middle of the fabric.
Most often the top tension is adjusted. Only when the top tension cannot achieve the correct thread tension will the bobbin tension be adjusted. Bobbin tension is very sensitive so make sure to mark the original setting first and make only 1/8 to 1/4 turn changes at a time.
Understanding Thread Tension Threads
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