First of all QUIT PLUGGING THE MOTOR INTO THE WALL SOCKET, YOU'LL BURN IT OUT! Of course it runs when plugged directly into the wall socket, it's getting 100% power.
A sewing machine motor must be plugged or wired into a controlling device. In other words the foot controller.
Most Japanese machines use a cord set like this:
http://www.shop.sew-classic.com/Cord...143-SCE143.htm
You have one outlet for the light plug and one for the motor. They are marked. They outlets are wired differently inside. The one marked LIGHT is wired hot all the time, lights have a switch. The one marked MOTOR is wired through the foot controller so the motor can be controlled.
Just stop what you are doing and post some pics of the wiring on your machine. Otherwise you could burn the motor out.
Is it worth it to spend a few dollars to fix an older metal machine? Personally I think that's a silly question. If the machine is a good one, your money ahead to fix it. You CAN'T buy machines like that anymore.
And unless you are very lucky, you don't get perfection in a used machine for nothing.
Joe