How do you adjust the vacuum?
you dont adjust vacuum you adjust the idle mixture screw to obtain the highest vacuum reading you can get. what your trying to acomplish is to get the proper air fuel ratio at idle.
you need to hook a vacuum gage to your wiper connection on the intake manifold. as you increase the vacuum it will change the rpm.
[if no vacuum gage is available turn the screw in or out to obtain the highest idling speed. if engine runs too fast after adjustment re adjust idle stop screw. changing idle speed may require slight reasjustment of idling mixture.
prior to making any adjustments tune engine per specs. run engine to normal operating temp and let stabilize for at least ten minutes]
this is from my gmc maintenance manual. 47-54 100-400 models.
once you have the idle speed where you want it with the highest vacuum obtained you have reached your end point.
