kgg

My Take: Speed reduction is like a double edged sword. If you use a speed reducer for just reducing the machines sewing speed no harm will come to your machines internals. If you want / need the increase in torque to punch through something because of the additional available torque that a speed reducer will deliver. You run the risk of damaging / twisting internal parts that are not designed to take that load. The better option is then to invest in a different / better machine that is designed to handle that type of work. My attitude has always been if I need to use a speed reducer for additional torque benefits I am using the wrong machine. kgg

I would suggest a long arm in a used machine Singer 29k-72 or 73 the only difference being the end of the cylinder bed are slightly different. As for new there are to many clones available so just go with who gives the best price and after support. You might also want to contact the person who just up and quit and see what equipment they have that maybe for sale. @Wizcrafts should be able to provide good sound guidance. kgg

Very nicely done. kgg

As a note some people did converted their machines from 134-35 needle system to the 135 needle system. If the previous owner did the conversion then for leather you would need 135 X 16 X (correct needle size) and for fabric 135 X 17 X (correct needle size). What I have found is using a leather point needle will give a cleaner needle hole in vinyl then a fabric needle. Fabric needles are meant to separate the fibres of the material rather then cut them. Really depends on what size of thread you are planning on using and the needle system it is setup for. Your machine can accept up to V138 thread. The 190-22 if I'm not mistaken is a system 190 needle used on the older PFAFF 145 so what they are suggesting is a #22 needle which is good for V92 thread. kgg

Yes we do. Yes I known what you mean. The only "good" thing is when using one if you cut yourself you feel it before you see the blood. I almost never carry a pocket knife even through the son buys really good ones and then sharpens them so they are as sharp as a razor. You see the blood dripping on the floor before you feel the cut. Ask me how I know. I must as a giggle try using one to cut leather. kgg

You are right cutting tools is pretty vast. I have a fairly expensive brand name round knife which rarely gets used, maybe once in the last year. What do you mean --- $7 utility knife? I typically splurge and use the $1.25 CAD ones from the dollar store that come with a spare blade for cutting. If I'm cutting chrome tan probably 3 or 4 oz I use a electric fabric rotary cutter. Cutting heavy fabric or leather with a pair of scissors for me would be a no go no matter how good they are, old hands. kgg

I forgot to ask if you could provide a photo of how you have your top threaded. kgg

I think some of your problem is you are using Tex 270 (V277) which the machine is not rated for. The 341 / 1341 clones of the Juki LS 341 or 1341 are rated for a #24 needle which will accept Tex 210 (V207) but the machines typically will only sew that in thinner material. A lot of clone machines rate their machines on the max size thread you can stuff through the eye of the needle vs Juki rates their machine on what it can sew with the max size top thread and bobbin thread in max. sewing thickness. That is why you see the Juki LS-341 or 1341 rated for V138 thread not V207. Insert bobbin in the top cap and through the tension spring so the bobbin spins counter clockwise when you pull on the end of the bobbin thread. I leave about 4 to 6 inches of end thread dangling. Place the bobbin and cap in the palm of your left hand. With your right hand grab the end of the bobbin thread and lift the bobbin case of your hand. If there isn't enough tension the bobbin will just spin in your hand and probably fall to the floor. You should be able to lift the bobbin and case off your hand otherwise you have to tighten the little screw by turning it clockwise about 1/8 of a turn. You do not want to flat that little spring or unscrew the screw to much as if the screw comes out chances are you will never find it. Try again. When you get the tension just about right you should be able to lift it off your hand. Once you can lift the bobbin off your hand try giving the end of the thread a couple of quick jerks it should cause the bobbin to spin slightly and yet be lifted off your hand. kgg

@Cihowe I would do the drop test for tension on the bobbin thread. If it needs to be adjusted turn the tension spring screw a 1/8 of turn at a go. If you over tighten that screw it will flatten the bobbin thread tension spring and you will have to replace the spring. Once those have been over tighten they are never right afterwards and you will never get consistent bobbin tension. kgg

To many unknowns to start with. i) what model of sewing machine are you using?? ii) what thread size are you using?? iii) what size of needle are you using?? iv) is the machine threaded correctly??? Need photo's showing thread from spool to the eye of needle?? v) Did you do a drop test or use some other method of setting the bobbin tension???? My take: When you make a circle type seam manually without using a circle guide the sewing speed and controlled movements both have to be properly matched. Particularly with drop feed machines there can be a tendency to twist / turn / move the material to quickly. With drop feed and walking foot machines there is definitely points in the sewing cycle where there is no downward pressure on the material. If you move the material at one of those points you can drag up from the bobbin extra thread and get poor stitches. Those poor stitches do not necessarily appear at that stitch point but in the following stitches because there is so much slop / extra thread in the bobbin thread which then can become a self compounding issue. My observation: At your starting point of the seam you did not lock your stitches and the knot / loop is visible. The knot / loop should be buried in the leather which usually indicates a tension problem. If you don't lock your stitch it will cause a tension issue. Typically 3 stitches (4 holes) forward and then reverse back through those holes and then do your seam. Another method is start your seam the distance 4 holes in would be and reverse back 3 stitches then forward. The second method reduces the amount of thread in each hole by 1/3. kgg

I agree with @DieselTech talk to someone that does basic machinist work and have a couple adapters made. Pretty simple stuff. Also get them to make an adapter with 8mm internal threads in the top adapter so you even use the inexpensive dies that the Chinese type presses use like punches, etc. kgg

I think that would be the best way and maybe add a couple more to give it a little extra character. kgg

I do like your idea of modifying a motor to give you what you would like particularly if you have a motor on hand. I modified what is called a variable speed buffer to do sanding and burnishing. I would be interested in seeing your end result.

If to changed what appears to be a clutch motor on the used machine out for a servo motor which should cost about $200 the total cost would be about $600. That is of course assuming the used machine is in good working order and mechanically fit. If the machine does the stuff you would like to do then you would have saved $1300 which would go a long ways to other things like needles, thread, attachment, leather, etc. kgg

The Sunstar KM-380 BL-B can take a max needle size of a #23 needle which basically means you can use a max size thread of V138 ( Tex 135, M20 ) thread in thinner leather. Here is a link to the machines specifications. ( https://www.manualslib.com/manual/970676/Sunstar-Km-380bl.html?page=8#manual ) Here is a link for thread to needle size: ( https://www.tolindsewmach.com/thread-chart.html ) kgg