JUKI MO-6800S Series Engineer's Manual page 68

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine.
Trouble
21. Puckering (Mainly concerned
with double chainstitch)
Cause (1)
21-1) Needle 1-A)
21-2) Thread 2-A)
21-3) Thread tension 3-A)
21-4) Throat plate 4-A)
21-5) Thread cam timing 5-A)
21-6) Feed dogs 6-A)
6-B)
21-7) Presser foot pres- 7-A)
sure
21-8) Differential feed 8-A)
ratio
21-9) Thread amount 9-A)
Cause (2)
The needle is too thick.
The thread used is too thick.
Both the needle thread and looper
thread tensions are too high.
The throat plate has a large needle
hole.
The thread cam timing is too late.
The leading edge of the feed dog teeth
has been rounded off.
A difference in level exists between the
main feed dog and differential feed dog.
The presser foot pressure is not high
enough, providing poor ironing effect.
The differential feed ratio has been set
for gathering.
The looper thread amount is not
enough, causing excessively tensed
stitches.
Checking order and adjusting procedure
Use a thin needle as much as possible.
Use a thin thread as much as possible.
Reduce the both thread tensions to a minimum.
Replace the throat plate with one with a small
needle hole.
Advance the cam timing. Refer to the pertinent
Standard Adjustment.
Replace the feed dog.
Eliminate such difference in level.
Increase the presser foot pressure.
Set it for stretching. When stretching light-weight
materials, be careful not to cause the presser foot
to contact unevenly with the materials.
Bring the thread cam thread guide fully to the
front to increase the amount of looper thread.