Don't touch them if they have never been messed around with.
They are a real nightmare to set up properly as they are air bypass screws but the problem is that trying to balance them on 25 year old throttle bodies with worn seals and linkages is very difficult to do as every time you open and close the butterflies the vac readings will change again.
To do them you ideally need 4 gauges because as thermal says you will be continuously chasing the other 3 readings if doing them one by one.
Ino it's best to leave well alone as they are preset from factory, it's only when people mess around with them that further problems start which will affect the way the car idles and performs with excessive fuel consumption, irrational idle, over and under fuelling of individual cylinders which could be very bad if your running any kind of power.
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