Well,
I changed all the gaskets, the regulator diaphram.

Now it runs like clockwork. I suspect, the carby gaskets were all ok, the rocker cover gasket was a bit lumpy.
I gave the rocker cover a light lap with some 600 grit paper too.

It's great to hear thos YS's, I love the low pitch "snarl" they make as they run just above idle.

Thanks everybody for your help.

BT