Mustang 440, engine won't stop

[ABruso]

When i turn the key off on my mustang 440 (ford gas engine), the engine continues to putter and spits out a lot of black smoke, fuel. I know that the alternator belt is pretty loose, could the tension from the belt help stop the engine. Thanks

 

[SkidRoe]

The Ford engines put in Bobcats had an anti run-on valve in the carburetor. Yours sounds like it is running on.
Does your machine have such a thing? If so, yours is either defective or it has been defeated to allow the engine run without power to the valve.