Duerkopp Adler 550-12-12 Service Instructions Manual page 24

ATTENTION !
By extreme settings; e.g. shortest possible stitch length and largest
possible thread quantity (elastic seam) care is to be taken that a sure
stitch of the needle into the thread triangle can still be made.
By over-large lower thread quantities missing stitches can occur.
When, with the above mentioned settings, the stitch length is
increased by a greater amount then the thread guide must be set back
toward 0.
A jumping-off of the lower thread in the area X of the thread take-up
disc could otherwise result.
The lower thread would not, as necessary, be pulled back through the
thread take-up disc during the hook return movement.
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