[Rich in Tucson]

I think what you are experiencing is not a feature of the Zenith carb, but of modern gasoline. Your modern gasoline may have more or less alcohol in it than mine and behave differently in Zenith bowls; I adjust the float level in every carb to what works on the engine it is meant for, after testing jets for flow rates which can be controlled, not to some arbitrary measurement of gas level in the bowl. After time has passed and after whatever has been done to the metal and passages of carb bodies (which cannot be controlled), each has to be individually restored, if it can be. I would be interested in knowing the results of the same test in 8 more Holley carbs...