JUKI MO-6800S Series Engineer's Manual page 63

WARNING :
Turn OFF the power before starting the work so as to prevent accidents caused by abrupt start of the sewing machine.
Trouble
9. Triangle double chain looper
thread stitches are skipped.
The needle fails to catch the
double chain looper thread.
Viewed from
the wrong
side
One stitch is
skipped.
10. Triangle double chain needle
thread stitches are skipped.
The needle fails to catch the
needle thread loop on the
double chain looper, resulting
in the stitch skipping shown
below.
Defectively
interlaced loop
The needle thread
extends to the
adjacent stitch.
Cause (1)
9-1) The needle point is 1-A)
crushed.
9-2) Double chain looper 2-A)
9-3) Adjustment of looper 3-A)
9-4) Thread cam timing 4-A)
9-5) Thread tension 5-A)
9-6) Threading 6-A)
10-1) Double chain looper 1-A)
10-2) Adjustment of looper 2-A)
10-3) Thread tension 3-A)
10-4) Stitch length 4-A)
10-5) Needle guard 5-A)
10-6) Double chainstitch 6-A)
needle thread guide
Cause (2)
The needle point has been crushed and
got thicker and shorter.
The thread hole in the tip has worn, and
the looper thread does not reach the
needle as shown at right.
Excessive returning amount or longitu-
dinal motion will often cause this stitch
skipping.
The thread cam timing is too early,
causing the looper thread to slack be-
fore the needle enters a thread triangle.
The lower thread tension is very low,
and the thread is not stretched.
The area around the thread cam has
threaded erroneously.
The chain looper is too high and too
close to the throat plate, or has bad
shape.
The returning amount is not enough,
causing the needle to miss the loop.
The needle thread tension is too low.
The stitch length is as small as 1.5 mm
or less.
A needle guard is too high, and catches
needle thread loops.
The needle thread guide is installed too
high, and it fails to tense the thread.
Checking order and adjusting procedure
Refer to the clause relating to the needle point
crush.
Replace the double chain looper.
Refer to the pertinent Standard Adjustment.
Refer to the pertinent Standard Adjustment.
Increase the thread tension a little.
Correct the threading.
Correct the height of the chain looper by pushing
it down until it cones into contact with the stopper.
Replace the chain looper having a bad shape.
Refer to the pertinent Standard Adjustment.
Slightly increase the needle thread tension.
Slightly increase the stitch length.
Check whether the position of the thread cam is
too late.
Refer to the pertinent Standard Adjustment.
Refer to the pertinent Standard Adjustment.