Four Stroker, thanks for your reply. This backs up what another flyers thoughts were, that the YS70FZ is on the limits for the Hatori 530 pipe.
I've since replaced the pipe with a Hatori 720 short cooling muffler and ridged headed, as its less restrictive.
This could also solve the problems I was experiencing with the engine over heating in vert warm weather, despite there being more than ample cooling in the cowling.
Regards Mike.