VAC Advance for Dizzy + Carb Vacuum Stuff
 

Hey all,
I gave in the other day and deleted the rats nest as I figured it’s the easiest way around having a later model carb on an early model emissions system, but I did have one question.
I’ve capped off every vacuum port on the carb (not the fuel vent/return line thingy, I’ll be running that to the air cleaner), and so I was wondering if that means the vacuum actuated secondaries won’t function anymore, and if so, then is my only option to move to mechanical secondaries?
(By the way I have not deleted the vacuum diaphragm for the secondaries).

Then for the dizzy vac advance, Rad mentioned in his rats nest delete video that, if you wanted to keep your vac advance, run those two lines to one of the two ports, and I was unsure whether he meant one of the two ports on the carb spacer plate, or one of the two ports by the oil filler neck.
And lastly, what’re the benefits to having vac advance? (If any). Is it necessary/vital to have, or is it optional?

Thanks for your time everyone, hope my questions made sense and are clear :)

-Pascal

The secondares are not operated by manifold vacuum, they are operated by airflow through the venturies, which creates vacuum. (High speed air has low pressure, the venturies cause air to speed up as it goes through, which is also what draws fuel through the boosters) This is all internal to the carburetor.

The vacuum cans definitely should be hooked up. One or two of the ports in the carb gasket/plate assembly get ported vacuum. This is vacuum taken from above the rising edge of the throttle plates, so it sees NO vacuum at idle and SOME vacuum once you crack the throttles open. I always forget which is which but knowing what signal you are looking for, it's easy to find which one to connect up to with a vacuum gauge.

I used to play around with connecting both cans, or connecting trailing only. If you have a stock carb, connect both.

The vacuum source for the secondaries is internal to the carburetor, sourced from the venturis as peejay said. Capping off external vacuum lines will have no effect.

For the vac advance pots, 9 times out of 10 you want the second to front nipple on the phenolic spacer. I have found some carburetors where this nipple is not connected to anything internally, however it's rare. If it pulls vacuum at light throttle it'll work - you can either use a vacuum gauge or a long tube that you can stick your thumb over to confirm this. The nipple should then be connected to both vac pots on the dizzy. Just use a tee connector or something, nothing fancy.

Vacuum advance improves throttle response and part throttle fuel economy, that's really it. I'd certainly say it's worth having, but that's just my opinion.

Thanks for the info guys!

I think that’s just about solved my issues, except I’ve run into one other.

Disclaimer: This is a long explanation, sorry lol, so I hope everyone is able to follow along :)

The carb that came on my car is an ‘84 from what I can tell, but my car itself is a ‘79, so the previous owner was probably trying to find a new carb for it and chose incorrectly… the car, however, came with a ‘79 spacer plate, which had the factory gaskets ripped off. I tried my best using the gaskets from the rebuild kit to restore it, and made holes in the appropriate locations for the vacuum to still work. I then got a parts car in January, which had both the ‘79 carb and gasket plate. I was hoping to use the carb from it, but I’m unsure whether it’s salvageable at this point, as for example the throttle plate shafts are seized. I then decided to use the spacer plate, but for some reason only now have I noticed the vacuum ports won’t line up lol.

Here’s a comparison picture:

https://cimg5.ibsrv.net/gimg/www.rx7...d69471e49.jpeg
The two ports circled in red for the bottom plate (the at-one-time-‘79 and now-an-‘84 plate lol) both get air from the port neck with the red arrow, and the same thing goes for the green. I’ve circled the ports that I’m assuming give vacuum to the dizzy, but I’ve just been assuming that those are the ones.

Here’s the underside of the carbs:

https://cimg7.ibsrv.net/gimg/www.rx7...484d326b4.jpeg
The ‘84
https://cimg1.ibsrv.net/gimg/www.rx7...a422d9080.jpeg
The ‘79

I’ve been using a little hand pump to pump air through the ports and hover my face over the holes to find the airflow, and that’s how I’ve been finding where those channels go, but I’m just not sure which ones will connect to those slits above the rising edge of the throttle plate. Any thoughts?
If anything I can try to get the ‘79 carb restored, I just don’t have a job and can’t get one because of school, so I just can’t afford anything crazy lol.

Oh also I wish I did, but sadly I don’t have a vacuum gauge lol, and one last little thing, the car isn’t running yet, and if anything I can solve the vac advance issue through trial and error once I do get it running, that way I can more easily check for vacuum.

Thanks for reading :)

Alright, I think my problems solved lol, I’ve actually decided to just run the all-original spacer plate, using the port on the left in the green, and just figuring the rest out through trial and error.

Thanks again guys!