I didn't think they were supposed to work on capacitor start motors. They are recommended for universal type motors like drills and routers as long as you do not have soft start or an existing speed control circuit. I have heard of some members wiring across a burned out speed control circuit and using one of the boxes. MLCS sells them and you can look at the specs for them on their site.
Some of the better bits I have came with a speed chart for bit diameter and you could probably find it in a forum search too, I remember seeing it posted once or twice in the last year or so. If you're having trouble with burning on your cuts, sometimes it helps to slow down then too.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.