Just came across this thread and everybody has very good suggestions,but something that you might be over looking is the fact that the ZZ4 has no exhaust ports in the heads to heat the bottom of the intake and that is fine for a race car or a EFI injected car, But on a carbureted street car in cooler weather it really needs that heated crossover to vaporise the fuel. I can't begin to tell you how many vehicles I cured cold run problems on back in the carb days just by pulling the intake and chiseling the carbon outta the exhaust ports.
You might try dropping the jet size a couple of numbers and see if it improves.