Monitor windvanes are remarkable machines. They will steer you on your course with only a whisper of wind!
They perform this magic without question, but only ask that you balance the helm perfectly! If you do not, the force of the rudder may cause the ropes to unspool from the adapter and the whole system fails.
If you seem to have just a wee bit of weather helm that you can't trim away, try using a clothes pin to hold the rope on the adapter. If the rudder turns the wheel, the clothes pin will keep the rope from lifting up and coming off.
This is not a replacement for proper sail trim, simply a crutch to get you through a time when perfect sail balance seems to elude you.