kgg

Personally like the A&E Anefil Thread which is a Bonded Nylon. I have never had problems A&E threads, their threads are made in both the Canada and the US and you can, at least in Canada, buy direct from them. The Sunstop from Sailrite is also an A&E thread a Bonded Polyester. Bonded Nylon is ever so slightly thicker then bonded polyester so the breaking strength of bonded nylon is slightly higher. Bonded Polyester handles UV and bleach exposure better then Bonded Nylon. My experience is the price difference between Brand name threads and the unbranded stuff can be substantial. The main problem is the consistency and quality of the no name threads typically Chinese stuff can be all over the board. Avoid the frustration and spend the extra for a brand name thread. The thread and needle are the inexpensive parts of completing any of your projects so it's better to start off with good quality. kgg

Before you go fiddling around with the machine change your needle to match the thread. The needle should be a #24 or in thick / tough material a #25. A good reference chart for thread and needle combination : https://www.tolindsewmach.com/thread-chart.html kgg

In your very first photo their is blurred image of your main spool which appears to have some form of thread banding around the cone in the center in the center of the spool. Could you please post a photo of the main thread spool and how you have threaded the thread through the thread post guide. Is this a recent problem that started to happen when you changed spools of thread. The thread appears to be a Chinese brand of thread which I have found to create weird problems caused by poor manufacturing. I would also. i) Check all surfaces where the thread passes through or around for sharp edges or grooves and ensure the needle isn't slightly twisted towards the front or back of the machine. ii) Change the needle to a #21 as the thread maybe sized different then what it is labeled as. iii) Change to a Schmetz brand of needle. I have had better success with their needles iv) Change spools of thread as it maybe a bad spool. v) Remove the banding around the center of the spool of thread as this maybe causing inconsistent thread tension. vi) Make sure you have a distance from the top of the spool of thread to the thread stand guide hole that is twice the height of the spool thread kgg

I have never seen a data base that lists the model with a cross reference to the year of manufacture for the Juki machines. kgg

Just to add to what MtlBiker said. What size and type of needle are you using. Should be a #19 or #20 with a 135 x16 needle. Is the machine threaded correctly for the top thread path and the direction the bobbin spins in the bobbin case as it unwinds from the bobbin? Is the needle threaded correctly, the needle insert all the way and in the correct direction? Did you do a drop test on the bobbin and bobbin case to get the bobbin thread tension set correctly? A few photo's would help of how you have threaded the machine from the thread spool to the eye of the needle. kgg

Just to add to the confusion. i) V138: Bonded Nylon has an average manufactured thickness of 0.41 mm while Bonded Polyester has an average manufactured thickness of 0.38 mm ii) V138: Bonded Nylon has an average manufactured strength of 22 lbs while Bonded Polyester has an average manufactured strength of 21 lbs iii) Bonded Polyester handles UV and bleach exposure better then Bonded Nylon. kgg

The upholstery grade machines have about 1/2" presser foot lift but you have to subtract about 1/8" from that for sewing purposes. This means you should be able to sew about 3/8" of material depending upon such things as how tough, how sticky the material is, needle thread combination, etc. Yes there is a large price jump once you go above the capability of sewing consistently greater then 3/8". Yes Juki does have medium duty machines that are rated for 3/8" like the compound feed Juki DNU-1541"S" flatbed and the compound feed Juki LS-1341 cylinder bed but once again if you are consistently sewing in the 1/2" or greater you are probably going to run into the similar problems. The compound feed on those machines (needle. feed dog and presser foot) will help to move the material along much better then the ordinary walking foot machines. The price of those machines: Juki DNU-1541"S" about $1800 USD and the Juki LS-1341 about $5500 USD. If you can figure a way around reducing the seam thickness like skiving by hand or by machine to be around 3/8" or under then i) price of a new machine becomes reasonable ii) you may not have to replace your existing machine. kgg

I would first suggest taking a sample of the thickest, including seams, of material or leather to a machine dealer and test drive a few machines. I would in a flatbed suggest looking at compound sewing machines that have the minimum sewing capacity of the Juki 1508 NH but you maybe once again be at their rated sewing limit. Once you get into needing too sew 1/2" thickness you are really, I think, needing a Class 441 machine. That would in a flatbed probably put you into a Juki TNU-243U or a Juki TSC- 441 cylinder bed with a flat top attachment. Both machines are grossly expensive probably north of $10,000. For most the cost is prohibitive so I would suggest looking at machines similar to the Cowboy CB 243 flatbed or in a cylinder bed the CB4500 with a flat top attachment. The cost of those machines are probably in the $3000 to $3500 range. I would stay with Juki or clones as accessories, replacement parts are cheaper and more readily available then say PFAFF and Alder. kgg

Nice machine, excellent video. kgg

A couple of things: i) Bad spool of thread -- Replace ii) Poor quality thread -- Replace and buy brand name thread. To add to Wiz's recommendations consider A&E thread. I use this thread in the 16 oz spool size and have never had a problem. iii) Mounting arrangement of the spool of thread --- If possible could you post a photo of how you have your spool of the thread mounted. The distance from the top of the spool of thread too the thread stand guide hole should be at least 2 times the height of spool of thread. iv) Dark thread is usually stiffer. Bonded nylon is stiffer then bonded polyester v) Size of spool of thread can also effect how the thread reacts as it comes off the spool. vi) Larger spools have less problems with spring memory then smaller spools. More problems with 4oz then with 8 oz which has more problems then 16 oz. kgg

Another place people forget too oil is the other end of the bottom shaft that is accessed by opening the two round cover plates on the main part of the machine. Like RockyAussie said: kgg

The #23 size needle I think is better suited to V138 (T135) so I would try a #24 needle with your T210 thread as it would a larger hole for the thread to pass through and the loop to be properly formed. Also hold both threads when starting to sew but make sure your bobbin is not over filled ( 80 % of capacity) and the bobbin tension is adjusted. If the bobbin tensioner spring was over tighten or is worn out you will not be able correct bobbin tension. A couple of video's that may help with checking the threading, bobbin and the bobbin case for problems: i) Juki LU 562 Bobbin Case Removal, How to load the bobbin and lift the thread ( similar bobbin case arrangement): https://www.youtube.com/watch?v=zvEsOKqdPZ4 ii) Threading a Singer 111w155 Vintage Industrial Walking Foot Sewing Machine: https://www.youtube.com/watch?v=Bkuf0__HzD0 kgg

The spring, poor bobbin tolerances, etc. This is used to help determine if the problem lies within the bobbin hook shuttle. If by reversing the position of the bobbin stitches become better you know where the problem lies. kgg

Yes the quilting people do spend a lot on their tables let alone their machines where the sky is the limit. Most of use, me included, complain and ponder about spending any amount for a new machine. Some hobbies are more expensive then others. kgg

Sometimes parts become weak with use and having the thread spool off the bobbin in the reverse direction can overcome the shortfall. kgg