[Penthode]

Have you checked the carburetor "secret passage" between the idle screw and the carburetor throat? There is a small hole in the throat immediately above the throttle flap which is the idle passage. I had the same symptoms as you. After checking pretty well everything else, I removed the top section of the Zenith and removed the throttle flap and idle screw and then poked a straightened small paperclip wire down the hole to clean out the crud from the passage way. You should be able to easily poke it from the throat diagonally until you can see the end from where the idle screw fits.

This passage is easily overlooked and because it conveys the idle fuel air mix and not just fuel, it tends to get blocked.

Once the passage was cleared and my Zenith reassembled, my car was transformed.