JUKI MO-6800S Series Engineer's Manual page 62

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
8. Double chainstitches are
skipped.
The double chain looper tails
to catch needle thread loops.
Viewed from the
wrong side
Two stitches
are skipped.
Cause (1)
7-5) Amount of thread 5-A)
8-1) Needle height 1-A)
8-2) Needle 2-A)
8-3) Double chain looper 3-A)
8-4) Adjustment of looper 4-A)
8-5) Thread tension 5-A)
8-6) Needle guard 6-A)
8-7) Needle heat 7-A)
Cause (2)
Excessive upper looper thread is fed,
producing stack of thread.
Needle
Slack of thread
If the needle is not correct, stitch skip-
ping occurs even when the returning
amount of the chain looper is correct.
The needle is bent or attached with
wrong orientation. DC×1 needle is used.
The lower part of the blade point has
been deformed, and the blade point
fells loops.
Clearance or returning amount is not
correct.
The thread tension is too high, prevent-
ing formation of good loops.
A needle guard is too high, and loops
are crushed. The clearance is too big,
causing the needle to shake.
The thread breaks due to heat gener-
ated on the needle, depending on the
type of materials, number of plies, and
sewing speed.
Checking order and adjusting procedure
Slightly lower the looper thread take-up (left)
to decrease the amount of thread. Shorten the
upper looper thread guide (right) to decrease the
amount of thread.
Slightly raise the looper thread take-up (right) to
decrease the amount of thread.
If the thread tension is not enough, increase it.
Refer to the pertinent Standard Adjustment.
Replace the bent needle. (Remove the cause for
the needle bend.) Correct the orientation of the
needle. Use a DC×27 needle (with a better recess
configuration).
Replace the double chain looper.
Refer to the pertinent Standard Adjustment.
Reduce the tension. However, be careful not to
reduce the tension too much, otherwise unstable
loops will result.
Refer to the pertinent Standard Adjustment.
Use a thinner needle. Reduce the sewing speed.
Check the coolant if necessary.