kgg

Based on what you have listed and looking at your website. i) post bed (Techsew 830 and the Consew 710 UX ) ii) 1341 class cylinder bed Cobra 26 and Techsew 2750 iii) 441 class cylinder bed Cowboy 3200 I think from a versatility point of view I would go with a 1341 class machine. So you would be looking for either a used Juki LS-341 or clone or new Juki LS-1341 or it's clones. The ideal thing would be to go to a dealer and test a couple of machines with your items and then select the best machine. Keep in mind no one machine will do everything each will have their pro's and cons. kgg

Very Nice. kgg

If possible add a couple photo's of your thread path from the thread spool to the eye of the needle. kgg

Yes. Every time you change thread spools, manufacturers of thread, thread size, needle size change or even changing the color of thread from a white to black (black is stiffer) you need to recheck the bobbin tension. Cheap Chinese thread in my experience is usually of poor quality. I have even had a needle break because of poor quality thread that I using in a pinch. They ran out of thread part way through spooling the cone so they they just joined the two end by tieing a knot to finish the spool. The needle broke when the knot wouldn't go through the eye. To much bobbin tension and you wind up not having the bobbin thread being pulled up into the leather while to little tension and the bobbin thread will be on the top side of the leather. The needle is also important factor as too small a needle for the thread size there isn't enough room for the threads, top and bobbin, to exist in the needle hole and you can get skipped stitches. While to large a needle and the stitch will be sloppy at best. Getting the top and bobbin thread tension right is like two teams having a tug of war with the needle being in the middle. What machine are you using? kgg

From the Purdue University kgg

A good thread size to needle size can be found at ( https://www.tolindsewmach.com/thread-chart.html ). For V138 you need a # 22 in thin and either a #23 or #24 in thicker / tougher leather. The #20 will not provide the correct size hole to allow the thread to move through it properly. That will cause tension problems and slipped stitches. Also are you doing a drop test on the bobbin to get the bobbin tension close? kgg

Great education machine. Doesn't this defeat the purpose of using a machine if you are going to pre-punch the stitching holes? kgg

What size of thread are you using with what needle size? Usually I back stitch about four or five. kgg

To get some answers you need to provide some more information. What and how thick of material are you trying to sew? What size of needle and size of thread are you using? A couple of photo's how you thread the machine from the thread spool to the needle could also be very helpful? kgg

The 3 layers of 6 oz leather or 18 oz in total ( just over 1/4 " thick) with V138 thread should be able to be handled by almost all of the upholstery class flatbed machines like your Juki DNU-1541S and clones but most load the bobbin from underneath the machine. To get the top bobbin loading you need to go up to a Juki LU-1508 class machine or clone. The Juki LU-1508 machine also has a larger bed and a double tensioner and is an overall heavier duty machine. Another option would be to go with a upholstery class cylinder bed machine in the Juki LS-341 or LS-1341 (newer version) or clone where the bobbin is loaded from the top and a flat bed attachment. My opinion of needle flexing is it has a lot to do with the needle size selected for the thread size, the presser foot on the machine and the thickness / toughness of the project. A lot of times it usually be solved by going up a needle and at a slower speed to give better control of the project to prevent side to side movement. The speed of the machine with a servo motor still maybe to much and the machine can be further slowed down by installing a speed reducer pulley. What machines are you looking at (new or used) and what is your budget? kgg

Tell the seller the serial number is typically located on the bed directly below the Singer badge and adjacent to the pillar. A good video on locating the serial number on Singer machines: "Dating your vintage Singer sewing machine" kgg

The machine in the video is a Mitsubishi Du-115 which shows the markings on the handwheel to be 0 to 7 for the stitch length. In the video there is a larger distance between the 6 mark and the 7 mark I am wondering if yours had another handwheel installed at some point in it's life or the markings for 6 and 7 are worn off . kgg

I wouldn't hesitate. Those bases alone are worth that price and are hard to come. kgg

Which one, I missed that. kgg

Looks good. Another good place to check for needles and thread is Wawak.com. Example: i) Needles: ( https://www.wawak.com/sewing/needles/industrial-machine/schmetz-leather-industrial-machine-needles-135x16-tri-dpx16-d-10pack/#sku=smnl135118 ) ii) V92 Thread: ( https://www.wawak.com/thread/thread-by-brand/ae/ae-anefil-thread-tex-90-1100-yds/#sku=thd290be ) kgg

To help with this you could fashion up a edge guide mounted to a little table surface. Don't use a magnetic edge guide on the nose area as it will lift the bobbin up and fowl the bobbin stitches. kgg

I think there a lot of these machines are gathering dust in basements as not everyone wants to go through the rebuild, tinkering process. The major downside I think is someone who could really do good items with a proper machine may become frustrated with this machine and consider all sewing machines to be as frustrating only to move onto another hobby. Definitely. kgg

What I see missing on first blush, the top thread tension disks are missing and a broken needle. But the price is right and a good project machine. kgg

I think this is a good point for discussion. I think if the top tension is to much then it puts to much strain on the the take up lever and parts and if the bobbin tension is to much then the bobbin assembly is at risk of damage particularly if the machine doesn't have a safety clutch. I usually just adjust the top tension first if I am assured that the machine is threaded correctly, the bobbin tension is correct for the thread and the needle is new and correct size for the thread size / material thickness. What I do is: i) change my needle for a new project or after eight hours of use or the needle seems dull / struggling whichever comes first ii) refill my bobbin with new thread and do a drop test iii) test seam If I have changed the thread size or changed thread manufacturers I will: i) cut the old thread at the spool and tie the old thread to the new thread ii) remove the thread from the eye of the needle and pull the old thread with the newly attached top thread through to the needle from the needle side of the machine iii) install a fresh bobbin with the new thread size and do a drop test. iv) test seam. If the test seam shows the bobbin thread on top then I first back off the top tension and if the bobbin thread isn't pulled up properly then I increase the top thread tension. Do another test seam and if that isn't correct I adjust the bobbin tension by an 1/8 of a turn. kgg

Very nice work. kgg

When selecting the right needle for the thread I would suggest sizing the needle to the thread. On CowboyBob's website there is a good guide ( https://www.tolindsewmach.com/thread-chart.html ). Depending on how thick / tough the leather is you may find that you may have to move up a needle size or two so the loop is formed properly so the hook can catch the top thread. The 135 x 17 can work in softer thin leather but the 135 x 16 is better as the tip cuts through the leather rather then punching through. The 135 x 17 is meant for fabric where you want to separate the fibers rather then cut them. kgg

I like this one from A&E : https://www.amefird.com/wp-content/uploads/2009/10/5-Thread-Size-Comparison-Chart-2-4-10.pdf kgg

This machine is a zig zag bottom feed tailor machine. Link: kgg

Try Aliexpress there are some aftermarkets stuff available. Search for 106-RP machines as they are the knockoff's ( https://www.aliexpress.com/item/1005002823365308.html ). kgg

My opinion is that just increasing the torque and needle stroke isn't going to be enough. Getting basically 1/2" rope to be sewn properly isn't going to workout with the machine. I could be most definitely be wrong but I think you need a proper machine for this type of work. kgg