Congratulations on your 15-91. I don't think people realize what a fine (and forgiving) piece of machinery it is, or maybe it's because, even 60 years later, supply so exceeds demand that its price remains low. The 15-91 was the first machine I bought and taught myself to sew on. I consider it the best all-around straight stitch machine Singer made. (The 401 is my go-to and I like the elegance of the 201, but recognize the superiority of the no-nonsense 15.) The minor inconvenience of a bobbin case (I immediately file complaints about this under "Whining") is far outweighed by its thread capacity and versatility with just about every thread and fabric under the sun.
If you feel you've really over-greased it, a good wipe-down should be enough; I don't see how that would make the motor run slower, however. At the risk of stating the obvious, are you sure the bobbin winder is disengaged?
Last edited by Manalto; 03-09-2016 at 10:58 AM.