JUKI MO-6800S Series Engineer's Manual page 61

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine.
Trouble
6. Lower looper stitches are
skipped.
The upper looper does not
catch the lower looper thread.
(Stitch skipping at time of
crossing of upper and lower
loopers)
Right Wrong
side side
7. Upper looer thread stitches
are skipped.
The needle does not take the
upper looper thread.
Right Wrong
side side
Lower looper
thread
Needle
Upper looper
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Cause (1)
6-1) Threading 1-A)
6-2) Upper looper 2-A)
6-3) Lower looper 3-A)
6-4) Adjustment of the 4-A)
loopers
6-5) Thread amount 5-A)
7-1) Threading 1-A)
7-2) Needle height 2-A)
7-3) Needle 3-A)
7-4) Adjustment of the 4-A)
upper looper
Cause (2)
The thread has been entangled with a
thread guide. Threading has not been
done correctly.
The blade point has a bad shape, and
fails to catch the loops.
The clearance between the needle and
the back of lower looper are not correct.
The returning amount of the lower loop-
er, height of the upper looper, or clear-
ance produced at time of crossing of the
upper and lower loopers is not correct.
Too much lower looper thread is fed,
giving slack of thread.
The thread has been entangled with a
thread guide. Threading has not been
done correctly.
The needle, if positioned too high or low,
may fail to catch the upper looper thread.
The needle is bent or crushed, in its
point.
The height of the blade point is not cor-
rect, making the upper looper unable to
properly pass the thread to the needle.
The clearance between the needle and
the back of the upper looper is not cor-
rect.
Checking order and adjusting procedure
Refer to the threading diagram.
Replace the upper looper with badly deformed
blade point.
Replace the lower looper with badly deformed
blade point.
Refer to the pertinent Standard Adjustment.
Slightly lower the looper thread take-up (left) to
decrease the amount of thread.
Slightly raise the looper thread take-up (right) to
decrease the amount of thread. Lower the lower
looper thread guide and shorten the lower looper
thread take-up to decrease the amount of thread.
Refer to the threading diagram.
Refer to the pertinent Standard Adjustment.
Replace the needle. At this time, be sure to elim-
inate the cause for such needle bend or needle
point crush.
Refer to the pertinent Standard Adjustment.