I would’ve not thought that this would get to be a pain in the neck, but there you go. I have a new SVDA Distributor with a vacuum advance but my carburetor set up doesn’t have a vacuum port. Short of pulling the carbs off in drilling them it’s been suggested that I either tap in to the manifold or air cleaner assembly. I’ve seen some suggestion that tapping into the manifold, even though the number of builders have recommended, it’s a bit of a no-no.

Putting a vacuum connection on the air cleaner bottom plate near the venturi opening would be easy and that should give me a vacuum signal, maybe weaker than manifold, but I wanted to ask what you all thought about that approach before trying it.

Thoughts? Comments? Experiences? Dope slaps?

Thanks!

Dope slap. Unless you are using the port on the dizzy for retarding the spark then manifold vacuum won't work for advance.

Ported vaccum would be just below the throttle plate and as you describe would be a weak signal for dual carbs.

Get an electronic programmable dizzy like a 123 and forgetaboutboutit.

Happy New Year TOm.

Take this with a grain of salt since I am NO carb expert, but as you know, I'm also going the vacuum advance route. First, I've found a lot of conflict and bad advice on the Interwebs on this subject as I'm sure you have too. My carbs are drilled and tapped for both ported and manifold vacuum ports but have tubes only installed on the port side. These are located just barely above the throttle butterflies and I think this is the best location because it will be the first place to pull vacuum when the throttle is opened and therefore give the quickest (and maybe strongest) response for advance. My gut says that a port in the air cleaner would be pretty weak.

I'm pretty sure manifold vacuum is wrong for advance. Again, take this with a grain of salt, I'm just going by what I've read trying to filter out the dumb s from those who seem like they know what they are talking about. But, if you think about it, ported vacuum taken from above the throttle butterfly is going to increase as the throttle is opened and vacuum below the butterfly will decrease. That's what you want for advance. When the throttle closes, the reverse should happen so manifold vacuum is where you would hook up vacuum retard if you are using it.

I could have this all mixed up but that's what I've been able to cipher so far.

Thanks for the detailed response, and that is what I gathered from the various discussions too.

I guess the question is if some advance via a weak signal is better than none?

I'm leaning towards pulling a single carb off and getting it drilled and a port installed.

Thanks for the response Rob, and early Happy New Year to you as well.

I've got what I've got here, so hoping to figure it out and learn something in the process.

I'm, leaning that way too. But, might be better to have both carbs ported and add an anti-pulse valve in the vacuum line between them and the distributor. Best wishes for success.

One car has EMPI knock-offs that have a port, odd that the real Webers don't.

You are better to tap off the manifold than into the carb. With a progressive two barrel the vacuum will vary from front to rear barrel. @Tdskip

Interesting- had not considered that.

Okay, I think I need some schooling here because I'm not understanding how manifold vacuum could be better for advance. Following is my understanding so please tell me where it is wrong.

1. Advance helps during acceleration so should come on during partial throttle.
2. At full throttle, centrifugal advance takes over so the vacuum signal (and advance it provides) is not important.
3. Vacuum retard smooths and reduces emissions during idle when the throttle is closed.

Now for behavior of the ports and when they will receive a vacuum signal.

1. manifold vacuum port anywhere below the throttle butterfly either in the carb body or manifold itself.
- Vacuum will increase when the throttle closes as the running engine tries to pull
air past a closed butterfly valve.
- Vacuum drops when the throttle opens and pressure equalizes as air rushes in.
2. port vacuum just ABOVE the throttle valve
- Vacuum is near zero when the throttle is closed and little air is moving through
the venturi.
- Vacuum increases as the throttle opens and air rushes past the port opening.
3. the vacuum signal should be strong enough to fully pull the diaphragm in the vacuum canister to completely pull or push the advance plate in the dizzy. More vacuum beyond that doesn't provide any advantage but a weak vacuum could reduce how much advance or retard occurs.

So... while I can easily see that a manifold port would provide the strongest vacuum signal, it seems like the timing of the signal would be wrong for advance. I don't have my D-Jet throttle body handy so don't know where the advance port is in relation to the butterfly, but I can say that the Italian Webers I bought new that are already ported and tubed for advance and drilled for retard but plugged have the advance tubes installed just barely ABOVE the butterfly valves and the retard ports are below.

What am I missing? I'm planning to run both advance and retard lines so want to make sure I understand what is happening.

I think I found the answer to my question. The difference between ported and manifold vacuum only matters at idle. If you connect an advance canister to the manifold side, you will get spark advance at idle. If you connect to the ported side, you don't. I can't say I understand all the opinions over whether spark advance at idle is good or not, but that is the difference. I found this curve that shows how the two ports respond to throttle position.



It matters to me because I want to hook up both advance and retard as it was from the factory (only on carbs). This article on dizzies at aircooled.net contains this:

"DVDA = Dual Vacuum, Dual Advance Distributor. The same as the SVDA, but also has a Vacuum RETARD port. One vacuum port is for advance, the other is for an idle retard, which improved emissions a lot!"

Doing my part to reduce emissions is very important to me so that's why the decision.

It's as simple as this- my car (dual Dellortos) has both ported and manifold vacuum sources available. My SVDA works properly using the ported source and crappy with the manifold source. So, guess which source I use?

Ansbacher