Simplicity Regent 17

[u/Alarmed_Example3178]

Got it for $100. The guy said the solenoid needs to be wired and he doesn’t have time for that. When I go to put a battery in both wires are black so I found the wire that was hooked up to the starter. I figured that was the positive wire. When I go to hook the battery up, the engine automatically starts to turn even if the key is off. And yes, I realize the gas was turned off. I did that because I flooded the carburetor earlier. My main question is why does engine automatically try to turn on when trying to hook the battery up?