My Dad used to call this condition "starving for gas". It is a very good explanation of what's happening. Yes it's a lean condition, at least a momentary lean condition. Additionally excessively retarded ignition timing can have the same effect.
I would suggest that you check the ignition timing first. Once you are satisfied with that, take off the air cleaner and peer down the carburetor throat with the engine not running, but the fuel bowls full of fuel. Snap the throttle open and you should see a stream of raw fuel from the accelerator pump nozzle. If you don't see the raw fuel, then what is happening is a momentary "hole" of air that is not being richened by the accelerator pump, thereby causing a momentarily lean condition. I would expect the accelerator pump to be the problem. Since the car has not been driven on a regular basis, the accelerator pump diaphragm or piston, I don't remember which is on a Zenith, is probably dried out.