You would raise the fuel level to activate the main circuit earlier.  

Ted, all of that makes sense.  Since my carbs and engines are stock, the only thing that has changed (other than age and wear) is the properties of the fuel.  There's no doubt that E10 behaves differently than the leaded fuels these carbs were designed for so I'm not surprised that some straying from stock may be required.  I think I will go ahead and start experimenting with float settings.  I feel far more comfortable with that adjustment than altering air bleeds and other calibrated orifices which, once altered, cannot be restored to their initial state.  

Is it fair to assume that most carbs are operating on the main fuel circuit during steady-state cruise conditions from, say, 35 mph on up?

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

So would you raise or lower the fuel level to activate the main fuel circuit earlier?

While altering the fuel level does affect the rpm in which the main fuel circuit is activated, it’s the combination of air bleeds and emulsion holes that actually controls the air/fuel blend at the various rpms.  This can be seen on the dyno using a wide band sensor and if there is some irregularity in the A/F ratio throughout the rpm band, then the air bleeds/emulsion holes can be modified accordingly.  As a general rule, the lower placed emulsion holes will affect the A/F ratio in the upper rpm band while the upper holes will affect the A/F ratio for the lower rpm band.  There are typically two sets of air bleeds per venturi with one being for the idle mixture and the other for the main circuit.  Larger holes in these bleeds leans the mixture while smaller holes makes the mixture richer.  I have the larger cfm Holley carbs here with three air bleeds for each venturi and the extra air bleed is for a real transition circuit.
 
While the factory carbs do a good job on the engines they are designed to be run on, putting these same carbs on engines with different cubic inches, camming, or other variables will make some of these carbs a nuisance to drive.

Lorena, Texas (South of Waco)

Ted, I was considering those transitions slots/holes as part of the idle circuit because they are fed with the same fuel/air mix handled by the idle jets, idle air bleeds, idle restrictor, etc. and are usually positioned in the same bore within the carb casting , just higher up the throttle bore, as you said.

Here's why I'm asking: I have 2 carb'd cars. When each has its float set according to spec, one (my y-block) starts visibly emitting fuel from the main nozzles at 1400 rpm, trans in park. The other, a small block 260, doesn't emit fuel until more like 1800-2000 rpm. The first seems early to me, the second seems late. The first drives great but gets bad gas mileage. The second is fine on mileage but stumbles a little on the transition to main fuel circuit. So, I have been trying to decide whether simply experimenting with float level is sufficient to address the issues, or whether I need to be looking elsewhere. Carbs and engines are stock.

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

Most carbs have a specific fuel circuit to minimize the lean condition that occurs between the idle and main fuel discharge circuits.  This would be the transition circuit.  The Holley 4V model 2140 carbs (1953/1954) did not have this transition circuit but on the Holley model 4000 4V carbs (1955/1956) there was an attempt at this transition by installing brass tubes located under the primary venturies.  On that carb model, the manifold vacuum was used to actually help draw the fuel from the main circuit earlier with the use of those brass tubes.  This actually worked reasonably well without having to provide a separate fuel 'transitioning' circuit.  On the model 4150 carbs (1957) the transition from idle to main circuit is taken care of by having separate fuel slots in the primary throttle bores in conjunction with the idle feed holes.  As the throttle blades open, more of the slot is exposed thus providing additional fuel at the smaller throttle openings.  Once the throttle opens enough to provide adequate air flow through the venturies, then the main circuit takes over.

Lorena, Texas (South of Waco)

The accelerator pump provides enrichment to compensate for the fact that, when the throttle is opened more quickly, the volume of air entering the engine changes instantly whereas the flow of fuel (being heavier than air) takes slightly longer to catch up. If the throttle was only ever opened slowly, we could get away without an accelerator pump.

The change in vacuum signal is indeed what pulls the fuel out of the main nozzles, but if the fuel level in the bowl is higher or lower, the strength of vacuum signal needed to pull the fuel out will be lower or higher, respectively. I'm wondering if there are other variables, such as mixture screw settings, jetting, or perhaps ignition timing, that would influence the point at which the main fuel circuit comes into effect.



1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

I would think the accelerator pump provides the transition from idle to main circuit. Both the timing and pump shot duration is tunable somewhat and the method differs depending upon carburetor manufacturer.

I always assumed it was a change in the vacuum signal as the plates open. I don't think float level as an effect but I'm not a carb expert, just general knowledge.

I wouldn't think so otherwise the engine will die if idling for a long time...  



"Came too close to dying to stop living now!"

As the throttle opens, all the downdraft carburetors I've worked with make a transition from an idle fuel circuit to a main fuel circuit. When it is on the idle circuit, there is no visible fuel discharge above the throttle plates, but as the throttle is opened further, fuel starts to come out of the main nozzles.

I am wondering: for a given carburetor and engine, is float level the only variable controlling at what point fuel begins to discharge from the main nozzles?



1954 Crestline Victoria 312 4-bbl, 3-speed overdrive