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Hi guys I know there is a lot of older vaccum posts but figured I could post here and see what happens . Looks like someone added this nipple behind the carb. Not sure if it should be connected to somewhere on the stock Rochester carb or plugged. When i put my finger on it I feel strong vaccum and the idle drops a little . Plug it and adjust idle on the carb ?
On my cars (70 and 71 Cutlass) that's where vacuum for the power brake is connected, but with a different fitting (90º fitting connected to a steel line). As Eric said, cap it for now.
There is nothing magic about vacuum ports. There are only three types of vacuum sources: manifold vacuum, ported vacuum, and venturi vacuum.
Manifold vacuum is tapped off the runners and the space on the carb just below the throttle blades. All of these locations are functionally equivalent. Manifold vacuum is high at idle and low at wide open throttle.
Ported vacuum is taken from a port (clever, no?) just ABOVE the throttle plates. At idle, ported vac is near zero. As soon as the throttle plate opens enough to expose the port, ported vacuum behaves exactly the same as manifold vacuum. Ported vacuum was originally used for the distributor vacuum advance in the mid-1960s as an early emissions control. A variation of ported vacuum has the port a little higher up the throttle bore so it takes a larger throttle opening to expose it. This is sometimes called a timed vacuum source and is mainly used to operate the EGR valve in the pre-computer days.
Venturi vacuum is taken from a port in the narrow part of the venturi. Venturi vacuum starts low at idle and increases as the throttle is opened and airflow increases in the venturi. Venturi vacuum is primarily used to operate the diaphragm on a vacuum secondary carb like a Holley. (And THIS is why the Qjet is NOT a vacuum secondary carb).
Thanks I appreciate it . Any and all info is super helpfull. Trying to apply what I'm learning 👍🏻
Originally Posted by joe_padavano
There is nothing magic about vacuum ports. There are only three types of vacuum sources: manifold vacuum, ported vacuum, and venturi vacuum.
Manifold vacuum is tapped off the runners and the space on the carb just below the throttle blades. All of these locations are functionally equivalent. Manifold vacuum is high at idle and low at wide open throttle.
Ported vacuum is taken from a port (clever, no?) just ABOVE the throttle plates. At idle, ported vac is near zero. As soon as the throttle plate opens enough to expose the port, ported vacuum behaves exactly the same as manifold vacuum. Ported vacuum was originally used for the distributor vacuum advance in the mid-1960s as an early emissions control. A variation of ported vacuum has the port a little higher up the throttle bore so it takes a larger throttle opening to expose it. This is sometimes called a timed vacuum source and is mainly used to operate the EGR valve in the pre-computer days.
Venturi vacuum is taken from a port in the narrow part of the venturi. Venturi vacuum starts low at idle and increases as the throttle is opened and airflow increases in the venturi. Venturi vacuum is primarily used to operate the diaphragm on a vacuum secondary carb like a Holley. (And THIS is why the Qjet is NOT a vacuum secondary carb).
On my cars (70 and 71 Cutlass) that's where vacuum for the power brake is connected, but with a different fitting (90º fitting connected to a steel line). As Eric said, cap it for now.
