My weedeater lawnmower was running fine until I bent the blade. I replaced the blade and decided to replace the starter mechanism since the starter cord was also broken. Now every time I try to start it, it fires (like a backfire) and jerks the cord out of my hand. What's the problem and what should I do?

Pull the flywheel off and replace the broken woodruf key. I'll bet its sheared and the flywheel rotated off from its proper index point.