There is a vacuum port, RED Arrow, on the drivers side above the idle mixture screw on my Q-Jet 4MV, does this get a plugged or is it open.





Thanks

 

On mine thats where the vacuum advance hose plugs in....

 

Regards from Down Under

aussiejohn

 

That's a source for ported (timed) vacuum. Most like to run manifold vacuum to the vacuum advance.

 

Ported vacuum. Used for vacuum advance on cars with emissions. I am running my vacuum advance off the unported source for a better idle.

As to the original question, any and all ports you are not using should be plugged otherwise, you will be introducing vacuum leaks.

 

No vacuum ports ever get left open. You need to either use it or cap it.

The port you show on your early model Q-Jet is the ported vacuum source normally used for distributor vacuum advance on emissions controlled vehicles. From a performance standpoint, most engines can usually benefit from running direct manifold vacuum instead of the ported vacuum to the distributor (all other ports on the carb are manifold vacuum), but hydrocarbon emissions will increase by doing so.

As a note, the integral-choke Q-Jets built after 1975 have the noted port location as manifold vacuum - not ported. The ported vacuum location on a 1975+ Q-Jet is the nipple coming out of the throttle baseplate at a 45-degree angle on the forward passenger side of the carb.

 

Thanks Lars, I am ordering the correct hose and steel tubing for this port and will run it to the Vacuum Advance on the distributor. Right now the advance is running off the vac port on the choke side which was for the A.I.R pump that is no longer installed. I will plug this port. See drawing.

 

As noted, unless you are doing a "correct" restoration, or if you are very concerned about emissions, the engine will likely run better using the manifold vacuum port you are currently using. The ported source will run your timing retarded, your engine will run hotter, and you will lose throttle response.

 

Then it looks like I leave mine where it's at......

 

HI Lars, Rich from the NE Tuning event, I currently use the same ported vacuum in this example, if I were to go to manifold vacuum, would I use a port off the carb (where) , or can I go straight off the manifold.

 

It doesn't matter. Manifold vacuum at the manifold or at a manifold vacuum port on the carb are the same.