Singer used two carbon piles in its foot controller. Over the years, some of the carbon wafers in either or both piles break or powder allowing a gap/space between the rest of the wafers. If a sufficient number of these wafers break down, essentially all you have left is an on/off switch, because contact is only made (internally) when the controller is all the way down. Replacing the foot controller is the quickest in this case. However, the controller has a "full throttle" adjustment screw that if too loose or bent (resulting in poor contact because of shortened screw length) , results in this type of symptom. Either way, it is easily repaired. I pulled my 431G out of a scrap heap and after replacing one spring, has been working like new. IMHO, the best machine ever made by singer. (next to the 222K which I also have)
Kelly Romain, Ontario, Canada, June 2012