JUKI MO-6800S Series Engineer's Manual page 65

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine.
Trouble
From the previous page
13. Overlocking needle thread is
loose.
Cause (1)
12-4) Throat plate 4-A)
12-5) Presser foot 5-A)
12-6) Main feed dog 6-A)
12-7) Double hooking 7-A)
7-B)
12-8) Thread tension 8-A)
8-B)
13-1) Position of the 1-A)
needle thread take-
up guide and needle
thread guide
13-2) Thread tension 2-A)
13-3) Needle 3-A)
Cause (2)
A dent exists on the flat part between
the rear edge of the needle hole and
feed dog groove.
Dent
The rear pressure foot is indented and
not flush with the presser foot sole, so
that it cannot hold chain-off thread.
The leading edge of the main feed dog
is too sharp and cuts chain-off thread.
The needle thread is not drawn up fully
because of the insufficient returning
amount of the double chain looper or
cam timing is early or wrong threading.
If the feed pitch is too small, the cam
timing will be advanced.
The needle is bend or chain-off thread
runs back due to excessive needle
thread tension.
Both the needle thread and looper
thread tension are too low.
They are positioned too high, and the
thread take-up draws out excessive
needle thread.
The thread tension balance has been
disturbed.
The needle is too thin for the thread
used.
Checking order and adjusting procedure
Repair or replace the throat plate, since such dent
causes chain-off thread to slip out.
Replace it, or correct it to make it flush with the
presser foot sole.
Buff the leading edge of the main feed dog.
Refer to the related Standard Adjustment for the
returning amount of the double chain looper or
cam timing. See the threading digram for correct
threading.
Retard the cam timing.
Reduce the needle thread tension.
Slightly decrease the both tensions.
Refer to the pertinent Standard Adjustment.
Refer to the Standard Adjustment for the looper
thread take-up components, and increase the ten-
sion if necessary.
Replace it with a proper one.