I can't say much about the Zenith carbs either way, but you might consider checking fuel flow by pulling the carb. drain plug. The fuel should pour out in a steady stream, accept no other. Forgive me if you've already done this but it could save you some $ -- the Ns have three fuel screens and it's easy to have one plugged. First is in the tank, part of the riser tubes -- second is in the top of the settling bulb where it seals against the valve body, third is in the inlet elbow on the carb.
My neighbor once fiddled around with ignition, carb and every other thing on his Furguson TO-20 for a month or more, only to find out it was a plugged screen in the riser tubes.
Hope this helps,
Pete P.
Harborcreek, Penna.