I have an older (10-15 yr) Ryobi straight shaft string trimmer. I assume there is some sort of centrifugal clutch between the motor and the shaft. Said clutch seems to have seized as the head never stops rotating. Even when you pull the starter the head rotates, and if you rotate the head you can feel the engine compression.
So, anyone ever replace one of these? The fact that the engine and shaft are always coupled makes it a real beach to get started. But once started it works like a champ!