Try what Joe said. I rewired and cleaned a Safety pot motor that came with a filthy Singer 27. It was amazing how just plugging it in and letting it RUN changed the speed and pitch.
It was very sluggish in the beginning, when I first plugged it in. Even 30 seconds of running and then unplugging changed the start up speed after a couple of times. Yes, I was very hesitant to just let it run. That's why I did the shorter times. I've since learned a few things! :-)