repair sewing machine Tension Issue in Reverse, Bottom Thread Only, Thompson Mini Walking Foot

[HeliSeams]

I have a Thompson Mini Walking Foot PW400, essentially what the Sailrite LS-1 was made from.  I’ve had this machine for a few years, was my first walking foot,  it would sew decent; never had issues I couldn’t learn to resolve on my own. Eventually I started replacing many of the worn out pot metal components with new Sailrite branded stuff. In the past few weeks I replaced the Crank Rod (couldn’t adjust , Presser Bar Track, Rear Presser Bar Track, Actuator, and Feed Rock Shaft Crank. These were either worn with set screw grooves, or beginning to crack due to softer metal. 

At some point this issue developed. Now when I reverse stitch, the bottom shows bad tension issues. I can forward stitch cleanly, but somehow I lose top thread tension, or the thread is resisting too much when going around the bobbin. Pics of stitches are attached.
 

I use this machine in helibases across the West as I travel constantly for fire in the summer. I repair crew gear, come up with new solutions, or just tinker and make myself and coworkers things. That’s why the portable walking foot is a necessity for me, and sewing is a passion, and also why it’s very frustrating to not have this machine fully functional. And I really just want to understand why this is happening, I’m very invested at this point. 

What I’ve tried so far:

-adjusting bobbin and top tension all over the spectrum. No change after getting in sync for forward stitches. 

-altering top thread path, trying new spools, new Sailrite feet, generic feet, new retaining ring and spring cap, raised and lowered presser pressure, changed bobbin and case, changed out the entire upper tension assembly and positions. Checked for burs/ damage in entire bobbin area. No change.

-V69 nylon and V90 polyester with size 110- 140 needles. Mostly with 2 layers of 100d Cordura or webbing. No change in reverse issue. 

-timed and retimed the shuttle hook, shuttle housing, needle bar height, inner and outer foot rise and motion according to Sailrite specs. No change after getting everything dialed back in.

-emailed Sailrite. They just gave the basic timing vids I already have access to, and suggested a new spring cap. Replaced both with new, no change. 

-met with a repair guy, he suggested some different timing specs to try that were very different from Sailrites: Lowering the needle bar with hook meeting needle at mid/ upper scarf instead of just above the eye. Having the hook travel a few mm farther counter clockwise before starting motion to needle. No change, and even skipped stitches. I have since gone back to Sailrite specs. 

-raised and lowered feet, and their forward/ aft positions (few different configs). No change. 

- slightly moved rocker positions via connection bolt positions in slotted arms that control actuator and drive/ reverse. No improvement on reverse tension. 

-probably many other attempts at tweaks that aren’t listed here.

-watched the entire forward and reverse motion in slowmo many, many times. The only things I can figure now are maybe the fabric is pulling up slightly at needle hole on the thread uptake due to different feet position in reverse vs forward? Or thread dragging in shuttle housing area (maybe due to shuttle driver being too far left or right, or something else)?
 

I am obviously missing something here, I feel like I’ve tried everything and am losing hope and ideas to figure this out. I hope someone can father something from this long post, and I can post specific pics if needed, but the machine timing and tweaks happen everyday. 

Thanks in advance.

[kgg]

7 hours ago, HeliSeams said:

I have a Thompson Mini Walking Foot PW400

Couple of questions:

i) You say you are using V69 nylon and V92 polyester thread with 100-140 needles.

questions: Are you using system 135x16 or system 135x17 needles. Are you using the same size thread for the top thread as you are in the bobbin. Are you using metal or plastic Singer 15 bobbins?

7 hours ago, HeliSeams said:

Mostly with 2 layers of 100d Cordura or webbing. No change in reverse issue. 

Question:

i) How many stitches per inch are sewing at??

The 100d Cordura is very thin. If I remember correctly Cordura is similar to Ripstop. Getting a nice bottom stitch may require either more layers or a filler between layers to allow the bobbin thread to lay correctly. When I sew 1000d Ripstop I find i have to use about 5 spi with at least 4 layers and I use the system 135 x 16 which is leather point made to cut through rather then separate fabric fibers.

Any chance of a video of your machine sewing and showing your top thread path from the thread spool to the eye of the needle?

Also here is a link for thread size to needle size:

https://www.tolindsewmach.com/thread-chart.html

kgg