Donna,
I believe I did use new needles. At least I hope they were, as they were in a needle package. I'm not too good about throwing away needles like I probably should, lol. But I think these were new. Donna, thanks for the great encouragement I will continue working on it. :) I do think I have an incorrect bobbin, I'm trying to get a vintage one that matches a picture in the 1950's 15-91 manual. Perhaps this is the problem. Oh, there are so many possibilities! Lol.

I used to throw away all needles I hadn't bought my self, maybe unless they looked brand new in a packages. I discovered some sell needles individually from a large package, they just pin them in a piece of fabric and into a paper bag. Either way, I relalized I probably had threwn out brand new needles because they weren't in what looked to me original packaging. I found this neat advice to run the needle over the tip of your nail, if it catches it's worn and if it's smooth it's still fine. Running it over a nylon stocking or similar will catch the slightest of wear too.

In hard times when new needles were hard to come by, they ran the tip of the needle over a fine honing stone to take the burr out of the metal, and they could stitch away with the same needle. I'm not that frugal, but I try not to throw out perfectly good needles, just because I didn't buy them my self. I haven't seen them in a while, but I used to buy a type of needle with a ceramic tip, or some kind of extra hard metal, they were made to last much longer than regular needles. It would be such a waste to throw them out just in case. Does anyone find checking the needle tip for burrs and wear reliable? So far it has worked for me.

Ladies, I am excited! Today I am getting new needles. Schemetz sharps size 14 and Singer titanium needles size 14, 16, and 18. I am also getting new thread to try. Aurafill, and Mettler. Later I might try Masterpiece quilting thread.

I solved the problem! After hours of adjusting the tension and pressure foot pressure, my stitches are now even! I loosened the bobbin tension, tightened the upper tension, and loosened the pressure on the fabric.

Uneven Stitches on 15-91
 

I hope someone has a suggestion that will help. I am using a 15-91 in good condition to sew vinyl for boat seats. At a slow speed I get the 6 stitches per inch I want (and the machine it set for). However, the faster I sew a seam the shorter the stitches get. Is there an adjustment somewhere that I am missing?

Haven't had that problem myself even at full speed so i'd try adjusting your feed rate :thumbup:

Hooligan
Thanks for the reply....By "feed Rate" do you mean how I pull material though the machine or something else?

Yes DVM, i meant how the fabric is fed through the machine. Don't pull the fabric, just merely guide it through and should be good. :thumbup:

The only adjustment, that I can think of, would be to change the pressure regulating thumb screw. I always forget which way, so would practice on some scraps.

Since you are sewing vinyl, you didn't say what needle you are using, or what foot, or if you have tried sewing with thin paper against the feed dogs and the presser foot. Years ago, I made some things with fabric backed vinyl. I read that a roller foot was good or a teflon foot. I was able to get a roller foot at that time, but not a teflon one. Also, to put scotch tape on the bottom of a regular foot. Another suggestion was to use a leather needle. Putting tissue paper (the thin type for wrapping presents) over and under what you are sewing, was also suggested.

There are several places on the internet for tips for sewing with vinyl.
Janey - Neat people never make the exciting discoveries I do.

I have been sewing quite much vinyl with my Singer 201, and the feed system is similar to the Singer 15-91.

I should look for the following:
1) Reduce thread tension (both bobbin and needle thread tension).
2) Increase needle size in order to reduce thread tension.
3) Increase presser foot down force.

This was still not sufficient for me in all cases, and then I discovered, that a changed presser foot could help you a lot. You can find information about that in this video: https://youtu.be/rBIulDuhDDs

You may also be able to change the feed dog timing, because it can help you when sewing heavier fabrics on domestic sewing machines: https://youtu.be/rBIulDuhDDs