Brother LH4-B800E Service Manual page 120

* The end of the lower thread protrudes above the seam at the sewing start
Cause
Lower thread Lower thread retaining
retainer amount
Lower thread Lower thread clamp
clamp timing
* Seam lifts up and does not tighten at the sewing start
Cause
Upper thread Upper thread scissors
scissors gradual opening timing
Upper thread Refer to "5-3. Uneven seams 1)......Beginning of sewing (1)"
feeding
* Bird's nests form at the sewing start, and after 5 -- 6 mm the sewing pitch suddenly increases.
Cause
Lower thread Lower thread clamp
clamp timing
Beginning of sewing (2) Lower thread lifted up
Check
Adjust the lower thread
retaining amount.
Adjust so that the lower
thread clamp plate
opens when the feed
mechanism moves 6 --
7 mm.
Adjust the lower thread
clamp opening timing.
Beginning of sewing (3) Seam lifts up
Check
Adjust the gradual
opening timing of the
upper thread scissors.
Beginning of sewing (4) Uneven sewing pitch
Check
Adjust the opening
timing of the lower
thread clamp.
Solution
Solution
Upper thread scissors gradually
start opening when the feed
mechanism moves about 1.5 -- 2.5
mm.
Solution
Advance the timing at which the
lower thread clamp starts to open.
LH4-B800E
5. Troubleshooting guide
page
4-19
1 - 2 mm
4-20
page
4-17
page
4-20
5 - 5