[NickRCarter]

Problem solved !!!
Whilst doing the run referred to in my previous post where I used a bolt to force open the secondaries after 3/4 throttle had been applied. I noticed that the paper clip on the secondary actuating rod attached to the diaphragm had only moved 1/4"
So I checked and double checked everything again on the carb and discovered that the throttle cable since installation had "grown" very slightly and coupled with post holding the outer cable at the carb having slightly moved on its fixing, full throttle was being prevented by about 1/4". Just enough to effect the flow through the venturi and degrade the vacuum being generated for the secondaries.
Today a run out showed the secondaries opening FULLY with the associated vast increase in performance.
Thanks everyone for your inputs, which have been greatly appreciated and helped a Guy on a sharp learning curve.
Now where's that performance meter..................?????
Cheers,
Nick