I think it is a stripped servo arm. I run the JR650M on my truggies and it works great. One thing you can look for is when you turn the wheels from left to right, look and see if the screw that holds down you servo arm to you servo and see if it is turning and the arm not turning. That is usually what you will see if the arm is stripped. The plastic servo arms strip out very easily and I replace all mine with aluminum arms and eliminate that problem. Also like mazdab2600 said some different companies have different amount of teeth on there servos and you have to get the right arm to fit them.