Quade

Damn I have one of those. They're fantastic for cloth. It's not a good machine for leather. I'd pick it up if it was close.

PU isn't very flexible. So if you have small pulleys, smaller diameter is probably better. I bought a couple different diameters but my pulley was simply too small to use them. Instead I picked up a 2L conventional belt which is about 6 mm. My setup is unusual. I don't expect you'll have the problems I had. I was trying to mate a "Sailrite Worker-B" motor to one of my machines.

I meant 3L = 9.5mm

When you buy the PU belt it's one long piece, longer than you need, so, you can wrap it around the pulleys and line it up that way to get the length. Then cut and join. You could cut the old belt and measure it. I'm using a 3/8" automotive belt on one of my machines. Picked it up at the autoparts store. I believe it's a 3L. Each L of a belt size seems to be 1/8" which is a little over 3mm per L. So 3/8" is 9.5 mm.

Servo's tend to have brakes built into them the make hand turning difficult. Try stepping on the pedal lightly, not enough to activate the motor, then see how easy it is to turn. I removed the brake from my servo motor.

https://www.amazon.com/PHYHOO-Head-mounted-Headband-Magnifier-Magnifications/dp/B00YX72CZ2/ I use this when I'm threading my machines.

It does look funny. I wonder if it's because he's not cycling the machine but just changing the stitch length? I'd like to see a full cycle of the machine manually advanced so we can see what the dog is doing. I consider it normal for the feed dogs to move as the stitch length is changed. I've never seen a machine that didn't have that. The stitch length adjustment is directly tied into both the feed dog position and speed and the needle rocker arm that moves the needle fore and aft.

I was wondering about that. On the machines I've taken apart, the manufacturers machined the shafts to have flats and somethings recesses that the set screws are supposed to ride in. If you follow their guidance, it's nearly impossible to install say a clutch or the hook gear in the wrong orientation. For example on my 211, the lower gear has a groove in the shaft that one of the clutch set screws sits in. There's two set screws so you need to note which one goes in the groove before you take it apart. With the set screw in the groove, even if it came a little loose, the clutch would still be held in the correct position. The hook gear side of the shaft has a recess that one of the set screws, the longer one goes down into. It's like a keyway on a crankshaft that keeps the crank pulley in the correct position on your car.

I haven't read the manual on this one. When you re-engage the safety clutch have you verified the timing marks (if any)? I'm wondering if it's re-engaging on a different spot on the gear. The shaft that drives both the feed dogs and the needle bar rocker is driven by an eccentric on the upper shaft. The eccentric seems to rock back and forth driving the stitch length and reverse mechanism which then moves the feed dog and needle rocker shaft. So hook timing isn't connected directly to feed dog timing like it is in a SInger 211. Meaning hook timing can go off from the clutch mechanism and not impact the feed dog or needle rocking position timing. Considering the condition of the lower shaft, I might make sure the clutch set screws are actually set inside any shaft cutouts. You just can't tell if someone tried adjusting hook timing by loosening the clutch set screws and turning the shaft. If it's not in the cutout, any jarring might make the shaft slip inside the clutch. Maybe the same check for the gear that drives the hook. I just don't believe you can assume anything when it comes to a machine that looks kinda boogered up. Might be worth painting some timing marks on the shaft/clutch and shaft/hook gear then if it breaks and goes off again, see if the shaft slipped.

That center shaft is what changes when you put it in reverse. You can see the linkage on the right side. I'm not clear how it works on this machine.

It comes down to knowing what you're going to sew and picking the right machine for your needs. I'd buy new if it was paying my bills. A hobbyist like me can afford to dick around with these machines because I'm not dependent on them.

I've gone down this exact path. My first machine was a Juki DDL-5550 Which is the Japanese version of the 8700. It was never used in a factory so, it's not worn out. I wanted to sew bags and some light leather. It's a great machine for cloth. The description of the machine is "light to medium duty cloth" and that's exactly what it's good at. It'll do 1000 d cordura too if you keep the thread size and number of layers down. It doesn't like larger thread sizes at all. I keep it below Tex 70. I bought a Singer 211 for heavier stuff, rebuilt it and it's the machine I'll use for heavier jobs. I'd love to have a newer Juki with unison feed but, I can't justify spending that money for what's just a hobby. The 211's are built like tanks and you can still get most important parts for them. At the very least you can buy one cheap enough the cannibalize for parts. The manuals for repairing and adjusting them are pretty terrible. Better then Consew but not nearly as good as Juki. There's lots of different variations on the basic 211 maybe 20 or more different part numbers so if you get one, make sure it'll do what you want. So my advice is buy more machine than you need so you don't get stuck with a machine that won't meet your goals.

Make sure you're using the right bobbin. Many of these older singers use a type-66 bobbin and not a type-15 you commonly see today. You will have problems if you use a 15 in a type 66 machine. First thing I'd do is look at the needle bar and see if you see timing marks on the needle bar. There should be two marks if there are any marks. The lower mark (close to the needle) is the timing mark for the hook timing, the upper mark is for setting the needle bar length. With the needle all the way down, the upper mark should just be visible on the needle bar. If it's not visible, you need to adjust the needle bar. There should be a clamp inside the front face of the machine you loosen to set the needle bar. You don't do timing till you know the needle bar height is correct. Then once it's correct, you advance the machine from the bottom till the lower mark is right at the needle bar bushing. At that point the hook should be right next to the needle above the needle eye hole 1/16". If it's wrong, you need to look in the manual for where to change the hook. It's different in different machines. If you don't have marks, you'll have to use a ruler for this. I'd have to look at a manual to know what heights to set it to. I'd suggest looking in Youtube. Seeing is better than reading my description.

The arm on the left hand side there with the slot in it. I believe that's how you equalize the stitch. You have to loosen some set screws to allow it to turn. I'd really recommend reading the manual.

The manual for my machine, a singer 211, mentions that some models have unequal forward and reverse stitch length by design. There's a limiter bracket installed on the reversing arm which limits both the forward stitch length and reduces the reverse stitch length. There's still an adjustment that would normally be used to equalize the stitch length on the arm that drives the feed dog shaft. Your machine is different than mine but there's typically an adjustment on the stitch length mechanism to equalize forward and reverse stitches.

I like your setup. I wondered how those integrated servo motors worked. I installed a "Worker-B" from Sailrite on the back of my Singer 211. Similar design goals. Your portable table looks terrific. Most machines seem to have an upper limit to the size of the thread they like. My Juki DDL-5550 doesn't really like much > Tex 70 or so. It'll sew but the tension is never perfect and it doesn't seem happy about it. With Tex 30-50 it works great. I wonder if you're running into something similar with this machine? When you use thinner thread, the thread you say works well, which route does it take? I didn't think this was something you normally needed to worry about. That the machine should route the thread correctly when you pull the bobbin thread up.

That's great.

I just took a re-look at the 206RB manual and now think I must have looked at the wrong manual. The mechanism is completely different than the 211. So my advice was mostly pointless. Sorry.

Stitch length is set by a mechanism that controls the feed dogs and needle swing. If it didn't do this before the "tune" I'd suggest someone messed up when tweaking the machine. 1 - I'd probably flip the machine up and try to move the feed dogs by hand with the foot up and needle out of the dogs. See if there's any slop in the movement. The feed dogs should be held tightly in place and shouldn't flop around. 2 - Set it to zero and longest stitch length and observe the feed dogs and needle swing. Does it change properly with no cloth or threading in place? You can measure the movement of the feed dogs to determine what the stitch length is set to. 3 - The fact it's intermittent makes me think something is loose or sticking. Off the main lower shaft is a lever which reaches into a joint which makes the feed dogs move as the machine is cycled. There's a clamp that clamps this mechanism to the feed dog shaft. The symptoms suggest to me this clamp might be loose. The reason I think that is that even with the intermittent stitch length, the needle and feed dogs remain in time. Which tells me that the problem is likely before this shaft. This shaft moves both the dogs and needle movement. The 206RB used the same mechanism for changing stitch length as the Singer 211 machine.

There's this spring loaded thing under the machine that connects to the main shaft, the one with the belt. That's the thing that sets the stitch length. When the stitch length is increased, the arm on this thing should get longer and move the shaft the feed dogs is on more. It's the same shaft that moves the needle bar back and forth. When you press the button on the deck and turn balance wheel till it catches, then move the balance wheel forward or reverse to lengthen and shorten the stitch. It's this spring loaded thing that changes to increase and decrease the throw of the arm. I'd probably prop the machine open and set if to zero stitch length (turning the balance wheel away from you till it stops) release the button. Then turn the balance wheel and look at the arm. It shouldn't move at all as the balance wheel is rotated. Then go the opposite direction, set it to max stitch length and observe the arm. It should move the most at that position. Basically it should go from centered with no movement of the arm to moving in and out with the balance wheel to move the feed dogs. If zero isn't zero and max isn't that max, it suggests the spring loaded thing isn't in the proper position on the shaft. There are set screws to hold the shaft and there should be a cut out on the shaft that one of the set screws actually screws down into. Meaning it shouldn't normally be possible for this mechanism to be in the wrong position. But who knows. My own 211 was a Frankenstein machine. It had been messed with quite a bit. There's a million adjustments on the machine and the manual isn't that great at describing how to set them. https://quadesimages.s3.us-east-1.amazonaws.com/Sewing/IMG_20221212_171209811.jpg

What needle size are you using? It has to match the thread. If the needle is too small, tension won't work right. https://www.tolindsewmach.com/thread-chart.html My favorite chart.

That navy manual is fantastic. Wish I had it when I took apart my 211. I have army version of the 111 manual and it's not nearly as well written.

I just want to say that there's a bunch of good information in this topic.

What needle are you using? Needle size is probably the most important factor when you can't get the tension right. If the needle is too small, the thread will drag in the cloth, requiring a huge amount of tension. https://www.tolindsewmach.com/thread-chart.html I use this chart which someone posted on this site. I once got gouged by an HVAC repairman because I watched him work. Still, I saw what he replaced and now have the tool and the knowledge to do it myself iif it fails the same way again. It was worth the expensive education.

I looked up an SA coded singer. It also wasn't in ISMACS. Turns out the machine is a Singer made in France around 1935. Not sure why they don't list it with all the other machines. I found a separate chart that listed it.