Very frustrating loss of bobbin tension mid-stitching, help!

[kgg]

8 minutes ago, medion said:

it seems like an increase in needle sized has likely resolved it in tests

A good chart for needle / thread sizing for future reference is: https://www.tolindsewmach.com/thread-chart.html

kgg

[medion]

I'm also noticing something else which is contributing to inconsistency - thread spooling off my thread cones is not smooth. I can feel it jerk, especially as it spools off the bottom, as if there is a minor stickiness factor in the cone thread. Is this a result of a bad batch of thread? Or?

[kgg]

16 minutes ago, medion said:

I'm also noticing something else which is contributing to inconsistency - thread spooling off my thread cones is not smooth. I can feel it jerk, especially as it spools off the bottom, as if there is a minor stickiness factor in the cone thread. Is this a result of a bad batch of thread? Or?

There are number of possible reasons so are:

1. Poor quality of thread and or thread tension on the spool during manufacturing usually found with inexpensive Chinese thread. Use a brand name thread to get consistency.

2. A lot of problems with thread happens with 8 oz spools particularly black color thread. Change spool and or replace with the 1 lb. spools.

3. The distance from the top of the spool of the thread to the guide on the spool holder needs to be at least 2.5 times the height of the spool thread. Adjust spool height.

4. To steep an angle from the guide on the spool holder to the first guide on sewing machine. Preferred angle is as close to horizontal as possible.

A photo of your thread path from the spool of thread to the first guide would be helpful in spotting other possible causes.

kgg