Take a manifold vacuum line off at idle, and notice how high the suction
in it is. Put a vacuum gauge on it if you've got one.
Take the port vacuum line off at idle, and notice how low the suction in
it is. Put a vacuum gauge on it as well if you've got one.
Next, with the engine off, spray some carb cleaner up inside the port
vacuum tap, and look inside the carb at where it is spraying into the
throat.
Heck, if you really want to see it all, get two vacuum gauges, two t
taps and a couple of feet of tubing. Put the gauges on both a manifold
vacuum tap and the port tap, and drive the car. Watch what the gauges
do.
What your going to see is that the port vacuum goes up and down in
direct relation to the air flow through the carb venturies in front of
the butterflies. The more air through the carb, the higher the port
vacuum reading. The manifold vacuum goes up and down in relation to
the engine rpms and butterfly position. Wide open throttle, zip
manifold vacuum, closed throttle on a high reving engine, very high
manifold vacuum reading.
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