JUKI MOL-254 Engineer's Manual page 59

Adjustment Procedures
Adjusting the stroke
1) Turn OFF the power to the machine while the sewing
machine is being actually engaged in sewing.
2) Turn the handwheel by hand until the needle thread is
caught by the hook and starts to turn in the inner hook,
then stop the handwheel before the thread completes its
turning motion (Fig. 3).
3) At this time, check how the respective threads are tensed
at portion . For the respective threads, loosen
adjust the stroke with
adjust it with for this side so as to slightly tense each
thread.
Tension
Far side : Loosen nut
This side : Loosen screw
for far side and loosen
and turn shaft .
and turn knob .
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Results of Improper Adjustment
If the stroke is too large, slip-off of
the thread is likely to occur.
If the stroke fails to match the thread
tension, the thread will excessively
flop.
and
and