The reason I use a 90, is that I know what it will do. An 80 will deflect faster, because the metal is thinner, easier to bend. Same thing with the 100, it will deflect later.
I feel like you have a bunch of tiny things out of whack.
I prefer my bobbin cases to be a smidgen looser than they say. I usually loosen the spring screw until it starts to unwind slowly, then stops by itself without having to give the thread a jerk. For me, that seems to lead to a more consistent result.
I think there's a chance the thread is mismatched to the needle. A known 50wt thread will possibly be able to boss an 80 needle around a little more than it would a 90 (make it deflect faster) You would also have a tendency to see more thread shredding with this combo in use.
The numbers on the dial are 100% dependent on assembly. I think this is your major stumbling block at the moment. I think the tensioner is still mis-assembled, but only slightly.
I've been avoiding putting up a tutorial about how to diassemble the top tensions, because of 2 things: 1. there's the TFSR site, though these tensioners are a little different, and
2. If I put it up, I'm concerned that I may not have the time to support all of the people who could have trouble putting it back together.
Here's the gotcha about the way the 411, 431, 401, 500 tensioners are built:
You want to adjust the tension to make sense -before- you put the number dial on. It's counter intuitive, but you would adjust the tension to have the ever so slightest perceptible drag on the thread and then put the "0" on the knob there, and put the remaining 2 pieces on to hold the number dial there. Gee, I hope that makes sense.
I think you'll find that if you're loosening the top and bottom tensions so much that the stitches look "OK" but the bobbin tension screw is almost out, that the stitches may look OK top and bottom, but if you separate the pieces of fabric, the stitches will gap. Not OK.
OK, the tension spring should drop to about 6 oClock when it's not resting on the little uh,... rest (or caught on the screw at about 7 or 8 Oclock)
You will also benefit from this Service Manual:
http://parts.singerco.com/IPsvcManuals/306W25.pdf
It's for the 401 (starting at page 125, but the first 79 pages are also relevant which is why I always post this link, and not the shorter one a lot of people post from the Slant Singer Yahoo group), but fundamentally, -most- of your machine is the same, not quite all though, so use some caution when things look a little different.
Page 132 addresses the tension spring and thread tension.
Let us know how you make out with it, and what you find. Tension is relatively "Easy" to fix, when you know what the symptoms are.
