On my machine, the stitch regulator pin is broken. It does not have the thin section that lives inside the spring (pointed at by the arrow), and its tapered end that contacts the regulator knob is worn. I slightly sharpened it, on a lathe, and it holds slightly better ever since.
When I go into reverse, the stitch length knob sometimes turns all by itself. I lived with this issue for quite some time because I can simply keep an eye on the knob and turn it back, but since I was ordering parts from College Sewing anyway, I bought the replacement spring, with the idea in mind to turn my own pin. The problem is that now that I compare the old and new springs, the new one from CS is shorter and weaker.
I cannot understand how a shorter and weaker spring can keep the knob from turning any better.