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Lock your throttle to a specific position, say 25%.
Then climb a grade that is becoming more severe as you continue.
Your vacuum reading will decrease as the grade increases, until you reach 0 vacuum and your speed drops as the grade continues to increase.
Until you change your throttle position, or shift to a lower gear, you will continue to read 0 vacuum and your speed will decrease as the grade increases.
Increasing your throttle position, as long as you do not increase enrichment, will have no effect on your speed, unless you downshift and get your engines RPM higher where it can produce more power.
Vacuum is a measurement of load.
The existence of vacuum indicates that you are lowering the effective compression of your engine by reducing the amount of fuel-air mixture that enters the cylinders. At 0 vacuum you have max compression. At highest vacuum you have lowest compression.
An engine idles because you are choking off the air and fuel supply until it will only run that fast (idle speed).
regards
Mech
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