So against my better judgement I tore the carb down again... then tore down the extra one that I had laying around, and built one out of two. The accelerator pump accumulator on my car's Carb body was a bit scabbed up and I was concerned about it leaking fuel when it ought not to. I also put in all the new diaphragms and so on so that the entire carb was in brand new condition.
I was finally at this point able to make sense of the tuning advice I'd been given. Basically the steps as I understand it is, scale back the idle speed screw until the thing's running as slow as you can get it ("sounds like a tractor motor"). Then adjusting the idle mixture screw in as low as you can get it, then back out until the engine stops speeding up.
I've not done enough KMs, nor filled up the tank to notice where I'm at with the fuel economy though. Prior to putting all new stuff in the carb, despite it running like a rat up a drain pipe, I was unable to tune the thing properly and I was up around 18~19 L/100KM (which as you might guess, is absolutely terrible, even for an early 1990s panel van).
I also noticed that it appears as though the "vacuum thermo switch" is not functioning, as I never (no matter how hot the motor gets) appear to have any vacuum applied to the two-stage idle solenoid (which does definitely work). I removed the switch from the circuit and simply applied vacuum all the time, and while it's temperamental to start, it appears to run much better for the moment.
I'm still quite happy with the build, if nothing else from a reliability perspective - I no longer have to two-foot the pedals at the lights in order to prevent the thing from stalling, which is a massive improvement. The power/acceleration gains are impressive, but virtually a non-issue for me - I rarely stomp on it. I'll be seriously disappointed if I spent several days, many hours of research, and about $100 in carby parts to have negligible improvement in fuel economy.
There's also the option of taking it to a place that has a sniffer and having it tuned correctly, but I really don't know if that's going to help all that much and I'm not sure I want to take the gamble at the current time.