JUKI MOL-254 Engineer's Manual page 211

2) With regard to sewing
Trouble
1. Slip-off of thread at the start of sewing
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Cause (1)
Stitch skipping at the first stich
Length of remaining needle thread is
insufficient.
Cause (2)
Cloth feed as against needle is improper.
Penetration resistance of thread as against
cloth is small.
Tension of the thread tension No. 1 is high.
Thread release timing is delayed.
Rising amount of the thresd tension disk No.
2 is small.
Stroke of the thread take-up spring is large.
Tension of the thread take-up spring is low.
Difference in height between the needle hole
guide and the counter knife is too high.
Needle thread tension is high and thread is
excessively stretched.
There is a scratch at the thread spreading
section of the moving knife.
Checking order and adjusting procedure
Narrow the cloth feed as against needle at the start of sewing.
(Change of cloth feed as against needle)
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 thinner needle. (Lower the count number of the needle used.)
Adjust the tension of the thread tension No. 1.
Adjust the thread release timing.
Adjust the thread release.
Adjust the thread take-up spring.
Adjust the thread take-up spring.
Adjust the height of the counter knife.
Adjust the needle thread tension.
Polish with buff the thread spreading section of the moving knife, or
replace the moving knife.