So, I've got more idea what I'm doing now and more idea what changing vents does and how emulsions work. Also more understanding of the relationship between mains/airs/vents/emulsions.
From the beginning then.
Idle sizing. This is where you can alter your mpg. Go as lean as the engine will put up with without farting and popping. This is the easy part. Dells or webbers double barrel carbs you remove the main stacks complete. Drive around. As soon as you're off idle you will be driving on progression holes, fed by the idle jet. These carbs have fixed idle air holes so you can only change the richness by changing fuel jets. In a way this is helpful - the idle airs are about the only constant thing! Doing this I ended up with pretty small idle jets 48's. I think it would go with even smaller ones but not much smaller. The general rule is 52-55 so my engine is "odd".
People say the idle mix screw has little effect on idle progresdion (driving) but I find it does and that the hole that feeds the fuel is like a first progression hole. Getting it idling as lean as it would cope with had an effect throughout the progression.
The smaller jets you fit, the less throttle they'll cover as you need more vacuum to pull through enough emulsified fuel. So maybe I could get up to 40mph on large idles and only 30mph on smaller, but as large ones would be running rich all the time there's no benefit to oversize idle jets. As you may surmise, smaller idles require the mains to operate earlier or you get left juddering from lack of fuel when the progression is done.
Vents.
Yep, bigger = better at high revs and smaller better at low revs, but it's more complicated in 2 ways.
The point the mains come into play. Smaller makes more vacuum so mains pull at lower loads (smaller throttle position).
The AFR curve at WOT through the revs.
Also the ramp between when the mains come in and WOT seems effected similarly.
I tried 30 - too too restrictive, too sucky, they caused a big rich overlap even with my tiny idles that I couldn't jet out, the idea being that at the dame smaller airs make a richer emulsion that needs more suck to get going, larger airs make a leaner emulsion that's easier to suck. Now... from low to high flows, WOT at high revs, the jets become progressively more of a restriction. By altering the proportion of air and fuel in the emulsion you can change how easy it is to suck and how potent it is. To cover my lean hole and lean off more at high revs I fitted bigger airs, up from 180 to 220 Then to get on target cruising or wot I fitted 155 mains. These are big jets for the venturis bit with the other sizes I couldn't get near so I'm suspecting that different emulsion tube might be more in the ball park. Never ending...
34 - with these I could jet for good WOT, but say 1/4 to 1/2 pedal was 15afr, which runs very hot, about as bad as it could be. Larger jets that cruised like this at 13.1-13.5 were much too rich at WOT.
32's have a flatter ramp but still a hole and still a ramp. I further adjusted that with combinations of main and air jets which maybe could have been done with 34's, but only by making a mahoosive air jets and and using too large main.
I created a lean hole by fitting much smaller idle jets. It wasn't there before I started. The vents have zero effect on the idle jet size or the idle mixture setting (which is just as well!).
The vents not only effect when the mains start to operate, but also the richness over the rev range. So do those pesky emulsion tubes and the fuel level, higher fhel level makes the mains richer as they start to perform. Not experimented with that.
They say that the top holes in an emulsion tube control the richness at low to mid throttle and blocking them makes this richer. Even hanging a wire through them has effect. This is kind of what I want and I think might allow me to run more sensible sized jets.
If you just read that... you did? Really? Does it make sense?