That would be the timing ring on the back of the motor. It doesn't come off, but if you loosen the 3 screws that hold down the rear cover, you will be able to move the ring. Clockwise takes timing out of the motor, counter clockwise puts timing into the motor. When you try to move the ring, it will not move by hand so you will have to use something to move it. I usually put a small screwdriver in one of the marks on the ring and push it.