garage door opener only turns for a few seconds each way, then stops

I have recently replaced gear and sprocket set on Craftsman 1/2 hp garage door opener. For some reason, even with chain (on top sprocket that connects to door) and limit gear and assembly disconnected, still only travels about 3 seconds in each direction. Tried by reconnecting all, applying correct tension, etc, and still doesn't work. Something in limiting gear, but I can't tell what. Any thoughts?