4300D Carb Vacuum Port Verification

[Moderator73]

Probably a really dumb question here...but want to verify that my carburetors "Spark Port" or Ported vacuum connection is the one in my picture with the Green arrow? That's how the vacuum lines were attached and I'm not very familiar with carburetors and would appreciate the confirmation. The reason I question myself is because there is another port on the front of the carb (with the yellow arrow) that is plugged. What is this one for, if anything?

This is an original Motorcraft 4300D on my 73 351C - 4V. I'm currently reconnecting all my vacuum lines and to say the least it's confusing. I've pulled a lot of great information off several threads in here that's helping me get these vacuum lines back to an original configuration, and so appreciate the collection of wisdom. Thanks

 

[Sheriff41]

That should be ported vacuum. It should have little to no vacuum at idle and the vacuum will increase with throttle advance off idle. Manifold vacuum will pull vacuum at idle.

You should check the vacuum diagrams for your engine as there were several variations on the 351 4V engines. I know for the 72 model year there was one configuration that called for manifold vacuum source while other configurations called for ported vacuum source.

If you're running a single diaphram advance you will most likely want to run ported vacuum, though opinions vary.

 

[Moderator73]

That should be ported vacuum. It should have little to no vacuum at idle and the vacuum will increase with throttle advance off idle. Manifold vacuum will pull vacuum at idle.

Thanks so much Chuck for looking at this. So are saying that the port with my Yellow arrow pointing to is the ported vacuum point on the carb? I just really didn't know, other than it's been hooked up to the other side where the Green arrow is for as long as I've owned the car. I'm putting it back together now but it's a long ways from being able to check the carb's vacuum while running because my interior is still completely stripped down.

I've had this car in pieces for the past 16 months to fix/upgrade several things and just about done with the work I've been doing on the front end & engine bay. Crossing my fingers that this will run when I get it all done, hopefully in the next 4-5 months.

Thanks again for sharing your thoughts!
Bob

 

[Sheriff41]

Bob,

As I recall, the one with the yellow arrow is the ported vacuum. Typically, ported vacuum nipple is above the throttle plates of the carb and manifold vacuum is lower, below the throttle plates. If you have a vacuum gauge you can hook to it, or even just connect a hose to the nipple so you can feel the vacuum. If it's manifold vacuum it will be very apparent. Also, if you're advance is connected to manifold vacuum you'll notice a decrease in RPMs when you pull the hose off of the vacuum advance unit.

@mrgmhale has posted vacuum diagrams for the 73 model years before, maybe he'll chime in here and shed his knowledge.

Chuck