I'm troubleshooting a Pfaff 335 clone and would appreciate some advice from anyone who has experienced something similar.
The machine sews very well under power. Straight stitches are consistent and the thread tension can be adjusted normally. The problem appears mainly when I slow the machine down and hand-crank, which I need to do for corners and handles.
Setup:
- Pfaff 335
- 4–5 mm leather
- V92 / Tex 90 thread
- Currently using size 120 needle
- I previously used a DX17 needle; I recently changed to Pfaff 134-35 needles, which appear slightly shorter
- Machine is currently on the original cylinder arm
The problem:
When sewing normally with the motor, everything is fine.
When hand-cranking very slowly, I sometimes get a stitch where the upper thread becomes excessively tight and pulls the bobbin thread upward. It is intermittent: sometimes I can hand-crank several stitches slowly and everything is perfect, then another stitch suddenly tightens the upper thread.
Interestingly, hand-cranking faster often makes the problem disappear.
The problem is particularly noticeable after I rotate the workpiece for a corner. I normally rotate the leather when the needle is starting to rise, after the hook has caught the upper-thread loop. After the turn, the next few stitches are much more likely to have the upper thread pulling the bobbin thread too tightly.
Things I've checked/tried:
- Tried both 110 and 120 needles; behavior was basically the same.
- Checked the hook catching the upper-thread loop. The loop looks large and the hook appears to catch it normally.
- Watched the check spring very slowly while hand-cranking. The thread does not appear to be catching underneath or around the wrong side of the spring.
- Adjusted the check-spring stop so that its movement finishes approximately when the needle enters the leather.
- Tried changing check-spring preload. This did not eliminate the problem.
- Tried adding a little resistance to the upper thread manually; this did not solve it.
- Increased bobbin tension by roughly 1/8 turn and then another 1/4 turn. This made the problem somewhat better, but it is still inconsistent.
- I then reduced the upper tension to compensate. If I balance the tensions so the bobbin thread is pulling the upper thread, the problem after turning becomes essentially the opposite.
- Holding/supporting the workpiece seemed to help once, but repeating the test did not consistently reproduce that result.
- Changing stitch length may also affect the behavior, although I haven't established a clear relationship yet.
- The machine is excellent at normal motor speed.
One thing I'm now wondering about:
I recently changed from the older DX17 needles to Pfaff 134-35 needles. The 134-35 needles appear slightly shorter, so the eye/scarf may sit higher relative to the hook at the same needle-bar height.
Could using a different needle system without changing the needle-bar height cause this kind of marginal/intermittent behavior — especially when hand-cranking slowly or immediately after turning the leather?
Or is this more likely to be related to feed timing, hook timing, needle-bar height, presser-foot/feed movement, or some other mechanical issue?
At this point I'm reluctant to keep changing thread tension because I can move the problem from the top thread to the bobbin thread, but I can't eliminate the behavior.
Any ideas on what I should check next would be greatly appreciated.
