If you study the power piston, power circuit and power valve and how it feeds fuel, you'll see it can affect many things...including idle as I'll explain in a moment. A stiffer power piston spring will keep your piston down longer and will make it need more vacuum to stay open. This will make everything richer. In your graph, it appears it is turning on/off. You're in NM, but we don't know where. If you're in Ruidoso, Taos or Red River, you're at a pretty high altitude and your vacuum at idle might be as low as 11~14...depending on the condition of your engine. If you're in Roswell, you've lost a lot of that elevation and your vacuum might be 15~16, but not higher and maybe measurably lower. So, depending on where you are you might not have too much vacuum from the start. If that is the case and if your Rochester is losing vacuum from typical Rochester problems (a completely different conversation), your power piston won't be working nearly as well as if you lived in New Orleans. As for the size of your main jet, I'll return to that in a bit. You might also remember with gasoline (good old gasoline) the stoich was 14.7, but with ethanol gas it will be less than that and if you're using 15% ethanol, it will be less still. Most data I've seen says to figure 13~13.5 or so for 10% ethanol and more like 9.5 or 10 for E85. The reasons involve the hydrogen/carbon bonds in the fuel and this can and will change between "winter" blends and "summer" blends. So there are a few factors to consider.
Because of the design of the B series, the fuel level is important in determining whether you run rich or lean. As you'd expect atmospheric pressure plays a part here (you probably have less and maybe considerably less) and the level naturally affects the distance the fuel has to travel for the idle circuit and also the main circuit. Higher fuel level = richer. Lower level = leaner.
In the B series, the idle scheme is simple and given the right circumstances will work well. At idle there isn’t enough air flow to make the venturi do anything, so the carb relies on the main jet, a vacuum passage, an idle tube in the float bowl, four air bleeds, discharge ports and a mixture needle. Vacuum pulls fuel up through the float bowl via the idle tube and into a cross shaft a couple of inches below the top of the carburetor in which air is mixed with fuel via 3 air bleeds and also an internal air bleed that is sort of hidden. Then the mixture is drawn back down through a passage in the fuel bowl and fed into the engine below the throttle valve. There are only 3 things you can change or adjust in a correctly working idle circuit: the size of the main jet, the level of fuel in the bowl and the position of the idle adj. needle. All the air bleeds and idle tubes and other things are fixed.
Located above the idle discharge hole are other discharge holes known collectively as the "off-idle" ports. They're activated as the throttle is opened, drawing air across them and increasing the same fuel/air stream already in play at the idle discharge hole. As the throttle is opened further and air flow increases, air pressure in the boost venturi begins to drop and fuel starts being sucked out of the main nozzle (which interestingly also works as one of the air bleeds in the idle circuit). This fuel travels through the main venturi down into the engine and this almost completes the idle/off-idle scheme. I say almost because the idle port will still be feeding fuel into the engine at this time. Rochester engineers believed this would create a smooth transition from idle to driving speed, and it wasn’t a bad thought. Or at least I'll say it works most of the time.
But unless the Rochester B or BC has been straightened and brought back pretty much to like new condition in other regards, I doubt it is ever going to work as well as a worn-out Carter. And I'm not just criticizing the Rochester because it is an easy target. The design characteristics were really that different, and where the Rochester's performance relies heavily on wear/tear/age and the number of "rebuilders" who monkeyed with it over the decades, the Carter YF isn't affected by those things nearly as much.
Personally I think before I began modifying too many things, I'd try different main jet sizes. If I hadn't sold all the Rochester parts I owned a few years ago I would be happy to mail you some. Here in Dallas, most of the B and BC models always seemed to work best with a jet size between .057" and .059", the most popular being the size of .058". In higher altitude, some will tell you to use one size smaller at 2000 feet, two sizes smaller at 5000 feet and three sizes smaller at 10,000 feet. How accurate that is, I couldn't say, but trying a jet sized 55 or 56 could accomplish what you're wanting and would be interesting. Also...even if you re-jet your carburetor for altitude, you'll still lose somewhere between 3 and 4% of your power with each increase of 1000 feet of elevation. This also is somewhat dependent on the condition of your engine. But let's say your engine produced 100 horsepower at sea level. At 7000 feet, if you figure a loss of 3% per 1000 feet, you'll have only about 79 horsepower. At 10,000 feet, you'll have only around 70 horsepower. So that does make a notable difference...especially in an engine designed to produce lower power from the start.
