1966 Chevrolet Corvair Starter Problem

I just replaced the starter and solenoid. When I turn the key and the starter engages, it will not stop even with the key turned off, until you disconnect the battery. Once the battery is disconnected, you can reconnect it and the starter is inactive. I tried it a 2nd time and the same thing happened. When I received my remanufactured starter the plastic housing at the rear of the solenoid was broken in pieces, so I bought a replacement and installed it. The large spring seemed weak in comparison to my original one. Could a weak spring be the problem? I can always use the old spring & solenoid if need be. What do you think? Thank you, Rudy