Lifting the piston leans the mixture without changing the timing; so it sure sounds like it is way too rich for some reason.
Are you sure the pistons are installed the right way around? There are tabs on the rubber diaphragm that should match notches in the piston & body, but sometimes they get overlooked. A diaphragm with a hole or crack would cause problems, too.
If I recall correctly, the ZS carbs on a 4A would have a "cold start bar", that would cause the mixture to be way rich if it were turned to the "choke" position.
Maybe the float is "sunk" (meaning it has absorbed fuel and become too heavy to hold the float valve closed when the bowl is full).
I don't recall offhand how the float bowl vents are routed on a 4A, but you need to be certain they are open to the atmosphere.