Hi, I have a White 763 (which is the same machine only straight stitch only). FYI it's also called the My Fair Lady Machines because they debuted at the World Fair;>
Anyway, there should be a motor plug and a light plug. If you put the motor cord into the light plug it will run constantly like you've stated. You should have a dual outlet with this machine marked "light" and "motor" and it sounds like you've got the cords switched. You can PM me to help if need be. I love my 763 for it's awesome straight stitch. Great line.