JUKI MOL-254 Engineer's Manual page 219

Trouble
8. Defective sewing when synthetic thread is
used
Cause (1)
Thread breakage due to heat
Hangnail of thread
Thread rises at the start of sewing.
Stitch skipping due to heat
Uneven tensed stitches due to stretch of
thread
Cause (2)
Sewing speed is fast.
Needle is thick.
Defective thread path
Finish of the needle eyelet is improper.
Cloth feed as against needle is improper.
Penetration resistance of thread as against
cloth is small.
Feed timing is improper.
Sewing speed is fast.
Needle is thin.
Fall of the needle thread loop
Sewing speed is fast.
Tension of the thread tension No. 2 is high.
Checking order and adjusting procedure
Decrease the maximum sewing speed.
Use a thin needle or a super needle for synthetic thread.
Use the needle cooler. (Optional)
Polish the respective thread paths with polishing bar or the like.
Replace the needle.
Narrow the cloth feed as against needle at the start of sewing.
Delay the sewing speed at the start of sewing.
Change the direction or position of the cloth feed as against needle
at the start of sewing.
Use a thin needle. (Lower the count number of the needle used.)
Change the timing.
Change the maximum sewing speed. (Electrical components)
Use a thick needle. (Raise the count number of the needle used.)
Attach the needle with its long groove slanting slightly (Approx. 20˚)
Decrease the sewing speed through the operation box panel.
Decrease the tension of the thread tension No. 2.