I had these intermittent flooding issues.
I tried everything until I finally got to installing a regulator.
I'm sorry if it looks bad, or doesn't fit into what you want, but everything else is really just a bandaid, trying to overcome the fuel pressure issue.
I know lotsa folks will say they run 7psi and have no issues. Heck I can do that too, but the one time it floods at a red light, you just don't want that to happen again. Having four bowls makes it even more of a possibility. Seems like once a needle and seat gets blown open it makes it more likely to happen again. I'd say the majority of dual quad issues I see are just too much fuel pressure.
Due to ethanol in the fuel, often fuel in the line will flash to vapor. This builds even more pressure and you get a massive rush of vapor/fuel pushing itself forcefully into the bowls. This is less of an issue with externally vented bowls, but still an issue.
For dual quad setups, the main keys that I use to get a clean idle:
-4.5-5.5psi fuel pressure
-Setting primary transfer slots fully covered, then back them open just a crack. Set idle with the secondaries
-Float setting: set primary bowl level with the roof. Set secondary level with the roof and crank it down two flats on the adjuster.
-1 turn out from fully seated on the idle mixture screws
When building 2x4 carbs from scratch, the settings above result in a successful test run 95% of the time with no other adjustments. I rarely even have to alter the idle mixture screws. If an engine desires a richer or leaner idle I prefer to make the change at the transfer slot or on the secondary idle feed restriction.
The holley books and common wisdom tells you to set the transfer slot square, or .020 or whatever. It tells you to set the floats level with the sight glass holes.
These carbs are installed backwards on 2x4 setups, so the sight holes aren't as accurate as they should be.
Why the transfer slot change?
Remember the idle circuit has one entry but two exits. One is regulated by the mixture screw, and one is regulated by the transfer slot exposure.
With one carb you have vacuum pulling at two slots and two secondary leak holes.
Add a second carb and you now have the same vacuum pulling at four slots and four leak holes.
The obvious conclusion is you'd need half or less of that slot appearing.
Jetting doesn't have the same effect as it works more via pressure differential than by vacuum.
Float level:
It isn't just about spilling out of the boosters or leaking into the idle feed. It's about circuit timing. The higher the float, the more emulsion air from the air well pushes into the main well full of fuel. The lighter that fuel becomes since it is aerated, the sooner it'll flow out of the booster.
Experiment to prove this:
Look into carb and slowly raise RPM until boosters start.
Raise or lower float significantly and repeat the test. You'll notice 500rpm or more difference in booster flow.
This is why float level is so critical even beyond the obvious "blowing past" a needle and seat.
If you look at Ford/Holley 3x2 carburetors you'll find how they worked this.
The primary carb has a very large High Speed bleed, .040 if I recall. So you have a lot of air headed into the air well. There is NO lower emulsion hole, the high hole is .027, the angle channel has a .027 hole, and there is a .027 hole from the upper angle channel drilled clear into the idle well. The outboard carbs are somewhat more conventional in setup, but the idea which shows out on an O2 meter and also while visually inspecting these, is that the fuel foam on the primary carb blows toward the booster super early. You have no massive flow, but you have very early flow that progresses as rpm rises. Of course by then the secondary carbs are in action and the fuel need is met without flooding out the engine.
Booster flowthrough effects idle as well. Too large of a HSAB compounds this. Don't believe me? block the high speed air bleed flow at idle and watch A/F mixture go lean. There is always flow through into the venturi from the boosters.
Why did I mention this? (Besides wanting to ramble) because if the float level is too high, either due to settings or due to uncontrolled fuel pressure, or if the HSAB is too large, it'll effect idle A/F ratio in a way you cannot control via typical idle fuel settings.
Or something.
