JUKI MOL-254 Engineer's Manual page 214

Trouble
4-1. Needle thread breakage
Cause (1)
Scratch in the inner hook
Thread is caught in the hook.
There is a scratch in the shuttle driver.
Clearance provided between the shuttle
driver and the inner hook is small.
There is a scratch in the needle hole guide.
Finish of the needle eyelet is improper.
Cause (2)
Scratch at section (Inner hook comes in
contact with needle.)
Scratch at section (Occurs when needle
is bent,or broken.)
Scratch at section (Inner hook is caught
on needle when it is removed.)
Scratch at section
Position of the shuttle upper spring is
improper.
Blade point section of the hook has
become like "R".
Position of the shuttle is improper.
Needle thread tension is low.
Thread take-up spring tension is low.
Length of the remaining needle thread is
long.
Cloth feed as against needle is improper.
Checking order and adjusting procedure
Polish the blade point with polishing bar or the like after smoothing
the blade point of inner hook with oil stone. Adjust clearance provided
between needle and inner hook.
Polish with polishing bar or the like after smoothing the scratch with
oil stone.
Polish with polishing bar or the like after smoothing the scratch with
oil stone.
Polish with polishing bar or the like after smoothing the scratch with
oil stone.
Adjust the position of the shuttle upper spring.
Replace the inner hook.
Adjust the position of the shuttle.
Adjust the tension of the needle thread.
Adjust the tension of the thread take-up spring.
Adjust the thread tension No. 1.
Narrow the cloth feed as against needle at the start of sewing.
Delay the sewing speed at the start of sewing.
Change the direction or position of the cloth feed as against needle
at the start of sewing.
Remove the scratch and polish with buff, or replace the shuttle driver.
Adjust the clearance provided between the shuttle driver and the
inner hook.
Remove the scratch or replace the needle hole guide.
Replace the needle.