why wont it turn

i just put my new 401 in my wildcat and it was turning over and it fired once i turned it off and let it sit,when i tried to start again its like the starter doesnt have enough torque to turn the engine.i checked all obstructions and other thing but when i try to start it turns real slow and will jump occasionaly.starter is smoking from not being able to turn.i put in a another starter not a new one but one that worked.same thing.when i take out the spark plugs it turns fine.please help.