Practically Classics Forum

Take the air filter off and see if that changes anything. There is very little adjustment on my zenith apart from rejectting, is it pulling air in from a dodgy gasket?

Accuspark are here

http://www.accuspark.co.uk/distributors.html

I fitted the electronic replacement 25d distributor and it was the best £60 I spent!

It sounds a bit like a partially blocked idling jet. The idling jet isn't just for idling, it supplies fuel at low speed and light throttle openings. The speed band you describe is the transitional stage from the idling jet to the main jet.