JUKI MOL-254 Engineer's Manual page 216

Trouble
5. Thread breakage at the time of thread
trimming
Cause (1)
Thread release timing is improper.
There is a scratch in the thread spreading
section of the moving knife.
There is a scratch in the shuttle upper spring.
Difference in height between the needle hole
guide and the counter knife is high.
Clearance provided between the needle hole
guide and the counter knife is small.
There is a scratch on the bottom face of the
needle hole guide.
Thread spreading timing of the moving knife
is improper.
Needle thread tension is too high.
Stroke of the thread take-up spring is small.
Tension of the thread take-up spring is high.
Cause (2)
Thread release timing is slow.
Rising amount of the thread tension disk No.
2 is small.
Thread is trimmed before the blade of moving
knife trims.
Thread is trimmed before the blade of moving
knife trims.
Thread is trimmed with the needle hole guide.
Checking order and adjusting procedure
Adjust the thread release timing.
Adjust the thread release.
Polish the scratch with polishing bar or the like taking care of the
blade section.
Remove the scratch.
Adjust the height of the counter knife.
Adjust the position of the counter knife.
Polish the needle hole guide or replace it.
Adjust the initial position of the thread trimmer cam and the moving
knife.
Adjust the needle thread tension.
Adjust the thread take-up spring.
Adjust the thread take-up spring.