JUKI MOL-254 Engineer's Manual page 215

Trouble
4-2. Bobbin thread breakage
Cause (1)
Bobbin thread tension is too high.
There is a scratch in the needle hole guide.
There is a scratch in the corner of the bobbin
case.
Adjustment of bobbin thread is improper.
Adjustment of the thread take-up spring is
improper.
Rotation of the inner hook is defective.
Loop end is not pressed.
Cause (2)
Stroke of the thread take-up spring is too
large.
Tension of the thread take-up spring is too
high.
Shuttle race face clogs with lint.
Shortage of oil amount
Checking order and adjusting procedure
Adjust the bobbin thread tension.
Polish with polishing bar after smoothing the scratch with oil stone.
Polish with polishing bar after smoothing the scratch with oil stone.
Adjust the lower plate.
Adjust the thread take-up spring.
Adjust the thread take-up spring.
Remove the inner hook and remove the lint.
Lubricate oil to the hook.
Adjust the loop supply device.