Mechanics, like doctors and lawyers and accountants , all think they're right. Has the rope snatched out of your mechanics hand? If your mechanic is as good as you think he is, Im sure he checked the valve adjustments. It doesn't take long to call roll.....there are only so many things that cause an engine to kick back. The ACR may operate when slowly turning the engine, but its not working when you yank it. Change the cam...that dog will bite you.