Thanks for the post Nosedragger.

The head has a port of 1mm, manifold is 10mm inside dia, my butterfly is IIRC 12.5mm, the transition taper is only in the ply isolation block which is 10mm. I overcame the impulse port issue by adding a nipple to the manifold and drilling the Walbro to accept another nipple, a small length of Tygon connects them up and it pumps very well.
I'm not familiar with "Fuel Drop Out", by that do you mean fuel spat back out of the induction venturi? I had quite a bit of that at full throttle with a butterfly of 14mm dia, the current carb appears to be performing much better, idle at 1100rpm, throttling instant to 6000rpm and returns back to idle very well.
I have noticed no drop in performance with my home brew manifold over the stock one. I binned the original carb, the barrel type as I screwed it up trying to lesson the return spring tension, I am very reluctant to go back to a barrel carb especially as I would have to buy a new one as my local garden machinery place has no 2nd hand ones with needle adjustment.

I am keeping my eye out for a Walbro with a smaller butterfly and twin needles but they are hard to come by 2nd hand........... I have a non existent budget to work with.

Could my rpm issue be the difference between a 8" and 6" pitches?

I have the stock exhaust on, that may be another source of restriction?