Pfaff 4874 Service Manual page 67

IRREGULAR LOOP FORMATION ON THE CHAIN NEEDLES (COVER HEMS) 29b
When the chain needles are in down position and going upstroke, the needles should pass
behind the loops. See cover hem stitch formation on page 30 and 31. However, it is possible
that some irregular loops will appear on the chain needle threads. In that case, the chain
needle threads will not be positioned correctly and will fall down to the right or left (Fig.1).
As result of the above, the following problems may happen.
1. Cover Hem (Narrow & Wide)
If "L2" needle falls into the needle thread loop (Fig.1-2). Check position of chainstitch
eccentric (0.5-1.0mm). See page 12 for adjustment. Check clearance between chain looper
and "L0 / L1" needle. If sewing result is the same, replace chain looper.
2. Triple Cover Hem
If "L2" needle falls into the Green needle thread loop (Fig.3). Check if the tension is
correctly set. Check the chain looper clearance and orbital timing "L0-L2". See page 12 for
adjustment. If sewing result is the same, replace chain looper.
3. Triple Cover Hem
If "L2" neelde falls into its neelde thread loop (Fig.4). Replace chain looper.
4. Cover Hem (Narrow & Wide)
If "L2" needle falls into both of the "L2" and "L1" neeldle thread loops (Fig.5).
Replace chain looper.
5. Cover Hems
If "L0" or "L1" needle thread is pulled to the right (Fig.6). Check and make sure that the
"L0" or "L1" needle is not touching the needle plate hole. When checking the above, use
new needle.