Thanks Jim,
I have considered tension as a factor. I have made sure they are never overtightened although I have had them tighter than I have them now. I have have two colleagues one a licensed helicopter engineer and the other a fellow pilot who has an HND in mechanical engineering. We are all used to working on, or doing prefight inspections of yaw control cables on helicopters which are kept tight. We are all scratching our heads as to the reason for my problem. The professional rigger I use says that the cables and blocks I have should allow a 180 degree change of direction and I only have 90.
I think the cables may be too thick but they are approved by the rigger man.
Thanks again for the thought.
Greg