Solved.

It was the ignition pickup being slightly too far away.

Spring in carb definitely not required with pipes from the crankcase breather via carb to manifold as shown


20/64ths without accelerator pump idles very well, but has to be set too rich for a good transition. The other engine WITH accelerator pump (and larger bore) does not suffer this problem.

I'll try a smaller carb, but I am sure it will restrict revs. I have a tacho these days!