Mattax said

If it was me, probably in this order.
1. leaks - leaks in the fuel supply to the bowls, and with a Holley, also the bowls.
2. High fuel level in the bowls causing some spillage/overflow. Remove top of air cleaner and see if the front or rear barrels look wet. Should not see wet on the boosters or throttle plates. If it has automatic choke, you'll have to open that to see down the front.
3. Normal venting from an open fuel system. Fuel bowl vents are open to the air cleaner and the gas tank is open to what? top of the fill tube?
It will be worse on hot days. Current pump fuel has a higher percentage of fuel that will vaporize between 100 and 130*F or so. The eth is part of the problem but its the entire formulation, so even eth free may or may not reduce that problem.
Some fuel distillation curves plotted in this post Video of boiling point of fuel


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For the basics, Chrysler's Master Tech does a pretty good job. But if you look through posts on a couple other forums I used to participate in, the smart money was on those that read and understood the industry publications. These include books by Obert, Taylor, etc. along with publicly available NACA documents, etc. When they discussed performance characteristics they cited, and provided charts from those publications. I've reposted some here at FABO. I'm just riding these guys coat tails. If you want to dig deeper, you want posts from people who know and respect the research and testing done forauto industry as well as racing and planes that had to engage in air combat.
Example from "Tuner" here
racingfuelsystems-Desired Carburetor AFR Characteristics At Different % Load


So far. Yes.
Spark plugs will give some insight into what's happening in the cylinders. Of course it will be a mix of reactions from a range of conditions, but if its extremely rich in any of them that still should show up.

If you want to mess with tune, make small changes one at a time. For example with the main jets, drive at 60 or 65 mph. If its not surging then you can try a little leaner.
If it seems rich at idle or off idle. Check fuel level in the bowls. Check transfer slot visible under the throttles when at slow idle position. With those established (adjusted) then some small steps can be made.


For the off-idle to work properly the primary side transfer slots visible under the throttle at idle need to be in the range of .020 - .040"
You can check with feeler gage or drill bits.
While you're at it, write down how many 1/4 turns in of the idle speed screw from just touching gets .020, 030, and .040. Then you'll always know how far open the throttles are.

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The more transfer slot exposed to vacuum at idle, the more fuel will get drawn from the slot rather than go through the idle port with the needle valve in it.
The reason for this is these carbs are designed to draw fuel from both the transfer slot and the idle port at slow idle.
When the throttle opens, already having fuel flowing from the transfer slot reduces the delay in increasing fuel to match the additional air.
If they are too far open at idle, then transfer slot happily supplies all the fuel the engine needs to idle.
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