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vacuum gauge/ fuel consumption
I not sure what the question is ,but you can buy a modern day vacuum gauge (stepper motor drive)which has a warning Led , that you can set the vacuum level at which it illuminates ,same as a shift light on a High performance Tachometer.
Real world long term road testing of various carburetor vehicles( I travelled 100,000 kms /year,during the late 90's through 2009) ,I found for best economy , avoid opening the power valve ,although you need to know the opening point(inches Hg)
If you open the throttle just above this opening point ,engine pumping losses are reduced as well as fuel consumption.
Best power is generally thought to be about 8%(12.5:1 afr) gasoline to air(power valve open) & best economy is about 6.25%(16:1 afr) gasoline to air(power valve closed ,although these figures are from non emission engines.
The power valve can be open without the secondary barrel(s)opening so careful use of a vacuum gauge ,while knowing the power valve opening point can reduce fuel consumption.
hope this helps
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