JUKI MO-6800S Series Engineer's Manual page 60

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine.
Trouble
5. Overlocking needle thread
stitches are skipped.
The lower looper fails to catch
needle thread loops. (Stitch
skipping of lower side)
Right Wrong
side side
Cause (1)
5-1) Lower looper 1-A)
5-2) Adjustment of the 2-A)
loopers.
5-3) Needle thread 3-A)
presser
5-4) Needle 4-A)
5-5) Needle guards 5-A)
5-6) Height of needle 6-A)
5-7) Needle heat 7-A)
5-8) Positioning of the 8-A)
needle thread guide
5-9) Threading 9-A)
Cause (2)
The blade point has defective shape
and does not catch needle thread loops.
The clearance or the returning amount
is not correct.
The duration in which the presser holds
the needle thread is not correct, and un-
stable loop result.
The needle is bent or improperly orient-
ed. A needle or DC×1 is used.
Incorrect height or clearance prohibits
correct guide for the needle. If a needle
guard is too high, loops are crushed
with consequent stitch skipping.
The needle has incorrect height and
does not properly pick up loops even
if the looper has a correct returning
amount .
Stitch skipping occurs before the thread
breaks due to needle heat.
They are positioned too high, and the
needle thread take-up takes too much
thread, producing too small loops.
The thread has been entangled with a
thread guide. Threading has not been
done correctly.
Checking order and adjusting procedure
Replace the lower looper.
Refer to the pertinent Standard Adjustment.
Replace the lower looper with badly deformed
blade point.
Replace the bent needle. Correctly orient and
attach the needle. Use a DC×27 needle. Use a
DC×J27 needle for a stretchy thread.
Refer to the pertinent Standard Adjustment.
Refer to the pertinent Standard Adjustment.
Refer to the clause relating to "1. Needle thread
breakage" due to needle heat.
Refer to the pertinent Standard Adjustment.
Refer to the threading diagram.