[KTL]

Pressure shouldn't be a tool IMO to adjust your AFR. Pressure should be adequate to fill the wells and keep them full for you. If you're too rich at 3.5, its not on account of pressure. The well is being filled and the float is rising to your desired level to shut off the fuel supply at the needle valve. Lowering the pressure is simply going to make the float fall more often, thus opening the needle valve more frequently. If you choke the pressure far enough to starve the well and your mains, that would certainly lean it out, but that's a very inefficient way to adjust for your problem.

Carb fuel pressure should not be confused with how most people think in terms of fuel injection pressure. Carb fuel pressure needs to be sufficient to keep the wells properly filled and that's all. Since you have a reservoir of fuel ("zero" pressure) in the wells that is feeding the carbs mains, fuel pressure is not a major factor in terms of mixture. You just have to ensure that the well is full enough but not too full.

If you're too rich, you need bigger air correctors or smaller main fuel jets. And note that the air correctors are typically a 3X change as compared to mains. Meaning a larger air corrector (more air) gets your 3 times leaner than a single main jet decrease (less fuel). Emulsion tubes are sort of a black art and i'd stick with what many say is a proven choice. Looks like those charts say F11 is a good choice for 2.7, so i'd stick with that.

Also your AFR numbers are contrary to what you state is your problem- too rich. If you say it runs better at 10, you are RICHER at 10.