JUKI MF-7900 Series Engineer's Manual page 188

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine
Trouble
2. Looper thread breakage
Cause (1)
2-1) Thread guide
2-2) Looper thread cam adjust-
ment
2-3) Thread tension
2-4) Thread
2-5) Looper avoid
2-6) Needle heat
2-7) Preventing thread winding
around the looper thread
cam
Cause (2)
1-A) Resistance produced by flaw, burr,
rust etc. in stitch tongue in throat
plate, looper, looper thread cam,
thread guide, thread tension disc.
2-A) Excessive tension applied due to the
incorrect position of looper thread
cam timing, thread guide
3-A) Too strong tension of looper thread.
4-A) Poor quality and weakness of thread.
5-A) Too strong contact of looper back with
needle resulting in thread breakage
6-A) In case needle heat is generated
looper thread's contact with needle
causes thread breakage, specially at
the time machine is at rest.
7-A) If the thread guide plate has been im-
properly positioned, thread breakage
will occur.
Check and corrective measures
Remove the flaw, burr etc. and process the
thread guide finish. However, replace such
parts as looper with the new part because its
shape is changed by being processed.
Refer to the standard adjustment figures.
Reduce the thread tension while checking to
see the tension balance against the needle
thread, top covering thread.
Replace it with good quality of thread.
Adjust longitudinal momentum of the looper
and make the looper contact the needle at 1/3
height from the bottom surface of the back of
looper.
Refer to needle heat in needle breakage.
Check the position and operation of the thread
guide plate. If they are not appropriate, re-
place the part with a new one.