Generally speaking, running the engine at a lower speed and at lower manifold vacuum (higher manifold absolute pressure) will produce BETTER fuel economy than spinning the engine higher at a lighter load setting. This is mainly for two reasons: 1) less "negative work" is done by removing the high vacuum from the tops of the piston during intake charging and
2) less frictional horsepower is lost due to the internal friction of the engine bearings, valve train, etc. Of course with a carbureted engine factors such the power valve / power enrichment circuit and optimal timing vis-a-vis engine octane tolerance at various loads also come into play.
I agree with this.
Back to carbs, I think there is something called a "transfer slot" that delivers fuel in between the idle ports and main jets. GM Rochester carbs I've messed with have these.
