I need help big timee!!.... I wil ltell you the story:

I went to the hobby-shop and got a new main motor because my old one was toastt, and i also got a 10 tooth car pinion ( only pinions they had ) and i set it up and tried to fly it. This is when somethgin weird hapeneddd..... I was flying, and noticed that it was difficult to just keep it in a hover, and tghe throttle was stoping and goiong quickly in mid flight, and the tail contorl was poor... After a 1 min, i landed, and picked the heli up and put more power on to see if the mainshaft was bent. It wasn't bent, but once i did that, smoke strted coming out of the motor and it smelled like burning plastic, i tried to turn it off, but nothing happened, it kept the throttle in half throttle alothough the stick was at 0. I had to reach my hand under and try to flick the batter off to stop the spinning blades of doom! I dont have a clue why this is happening. Is this car motor to powerful, or is it not ment for that type of use??? should i just buy thr stock motor again? Please help it would be appreciated!!