It's probably more correct to say that a larger than optimum carburetor can actually run lean. Regardless though you're right, this is not intuitive. The venturi effect is what atomizes the fuel, if the carb bores are too big for given conditions the fuel is not atomized efficiently and drops out of suspension. Then the tuner is forced to try and compensate by making non-optimal changes to baseline jetting and other important adjustments. This is why carburetor specialists always say the most important "adjustment" one can make is choosing the correct carburetor in the first place.

The vacuum or air velocity is what draws the fuel from the well, that's why the correct ignition timing curve (and fuel height) is important, because ignition timing directly affects engine manifold vacuum and consequently the AFR. At idle very little air moves through the carburetor, so manifold vacuum is used in a special idle circuit. Much driving is done just off idle, so getting the ignition timing right first is crucial.

Usually too you'll hear that it's always necessary to jet up when a dual exhaust or headers are installed due to better breathing and exhaust scavenging. The answer to that is "maybe"; the only way to know for sure is to measure the AFR, every engine is different, the carburetor and especially manifold all play a role, compression ratio, etc. A higher velocity through carb may just pull more fuel regardless and make it a wash. A wideband O2 sensor is very interesting to use and really see how changing a single component, or an operating point, changes something else, not always the way one might think!

Right, power valve should never be open at cruise. 4 to 5 mpg hwy? That's approaching the burn limit for gasoline most likely.

First thing make sure the distributor curve, mechanical and vacuum advance is squared away, and ignition has a fat, HOT spark. Can't tune the carburetor till that stuff is done and finalized. Next, are you tuning with a vacuum gauge? The numbers you mention aren't making any sense, it's way too rich at cruise and then, leaning out on power or acceleration. You need to use a higher # power valve if it is leaning out, not a lower one. Use stock heat range plugs, or a step cold, and stock power valve when starting with tuning.

It's important to know the steady level ground cruise manifold vacuum at 50 or 60 mph in high gear. Plumb the gauge into the cabin so you can read it. The idea is to choose a value that the PV is completely closed under steady level ground highway cruise conditions, regardless of speed, basically. If it's open, even a little bit, it will screw up finding the correct jet size. The correct PV for a given engine is based on both altitude and the average manifold vacuum of the engine. With a stock engine it's not difficult and a lot more forgiving than a performance camshaft.

When you are certain the ignition timing and fuel level in the carb is correct, the power valve is closed at cruise use the O2 sensor to get your jetting. Take it out on the highway and cruise at 60 mph on level ground and jet down. I run 15.5 to 16 AFR right in there. Look for evidence of "lean surge", you want to be just below that. There will be little to no load on the engine under this specific condition and the engine will tolerate leaner than "stoich" fuel mixtures without danger. This is where the vacuum advance comes into play, there will be a lot of ignition advance too, somewhere around 50° BTDC when the mechanical + vacuum advance is factored in.

Under power, under load, on acceleration, you cannot go lean or valves will burn. It has to be far richer than "stoich" in fact. I shoot for 12.5 AFR on acceleration. This is plenty rich and safe. Here's where it may be necessary to drill out the PVCRs ("power valve channel restrictors") after jetting is dialed in on a Holley type carburetor. The PVCRs are basically the two fixed jet size orifice under the power valve for the power circuit. Enlarging these with a pin vise just a couple thousandths will bring the AFR back to the rich side of things if necessary.

What you're doing here is "curving" the fuel supply for idle, cruise, power, and everything in between, sort of like curving a distributor. Remember the Power Valve just determines when extra fuel is provided, not really how much. It may be necessary to modify the power circuit slightly. But if the power valve # is too high it will enrich even at steady low load cruise, or maybe when driving into a stiff headwind, and bugger the AFR and waste impressive amounts of fuel. Can't get it with a truck, but lighter cars with V8 will smoke the tires and still get 25 mpg by carefully tailoring the fuel curve this way.

OK, sounds like you got er all figgered out then. /jk

15 to 16 AFR should be no problem at level ground cruise if ignition is straight. Jet down on the highway till it squeeks and fatten up the low end as required to bring the AFR under acceleration/power into line.

Yes, I'm familiar with those. They are good. Fuel injection engines run 18+ AFR on the highway. That isn't possible with carbureted engines, the cylinder variance of the charge isn't consistent enough across all cylinders, And it's only as good as the leanest cylinder in any setup, EFI or Carburetor. Also the display indication is an average of only 4 cylinders out of 8.

My slick runs perfectly fine cruising at 15.5 to 16.2 or thereabouts at steady highway cruise. I just had to hog out the PVCRs slightly with a pin vise so the power and acceleration AFR didn't lean out too far at wide open throttle. I didn't really get that deep into it, some guys will get into enlarge the idle air bleeds and high speed air bleeds and all kinds of stuff like that. The engine will almost cruise on the idle circuit when everything is just so, is kind of what they are getting at.

But that's how a carbureted V8 from the 50s or 60s in a mid size car or hot rod can get 25 mpg hwy and still smoke the tires around town. The last highway run out of state was getting just over 16 mpg with good gas which I expect is about the limit with heavy pickup gearing and the brick-like aerodynamics. I've since upgraded the ignition a fair bit and may play with the timing a little more.