JUKI LU-2800 Series Engineer's Manual page 127

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine.
Trouble
5. Loose stitches (Balloon stitches
or isolated idling loop)
Cause
5-1) Clearance between the opener and
the protruding section of inner hook is
too large.
5-2) Thread paths have been poorly fin-
ished.
5-3) Bobbin fails to move smoothly.
5-4) Thread is not well tightened in the
reverse feed.
5-5) Height of the feed dog is too high.
5-6) Feed timing is excessively advanced.
5-7) Hard-to-slip thread is used.
5-8) Bobbin thread does not pass through
the tension spring of the inner hook.
5-9) Bobbin thread tension is too low.
5-10) Bobbin has been wound too tightly.
5-11) The needle thread tension is low
while carrying out the reverse feed
stitching.
Checking
1-A) Check the clearance.
2-A) Check the finish of the respec-
tive thread paths.
3-A) Check the change of bobbin
thread tension.
4-A) Walking foot is not of top-divid-
ed type.
5-A) Compare the height with the
standard value.
* Use separately available parts below :
• 21144308 Needle thread lubricator
• B1114012000 Thread guide rod
• B91351230B0 Silicon oiler
10-A) Check thread winding condition.
Corrective measures
Refer to "2.-(5) Adjusting the opener".
Finish using a fine emery paper or buff.
Replace the bobbin or the hook with new ones.
Use the optional walking foot and presser foot.
Refer to "2.-(3) Height of the feed dog".
Lower the height by 0.2 mm than the standard
value.
Retard the feed timing.
Use silicon oil. (Refer to * below.)
Thread the bobbin thread correctly.
Increase the bobbin thread tension.
Decrease the tension applied to the bobbin winder.
Advance the horizontal feed timing.