Scottd

I just got my Quadrajet back from Lars and he noticed some issues that he felt was related to my vacuum advance. In the past, Ive hoked the vac advance up to the small tube that sits on the driver side, facing forward, just behind the throttle linkage. Lars has recommended that I use a full vacuum port, preferably directly from the manifold.

Lars also suggested that I use the long tube that comes out of the passenger side of the carb....

I currently have a 1 nipple port that I run my headlights from. I cannot find a 2 port fitting that will accommodate both size hoses. Can I split that line with a 'T' and plug my advance into it?

I guess Im a little confused because the port that Im using is the port I see all the manuals tell you to hook to, and I cant find any examples of the vac advance hooked directly to a manifold source.

edit- engine is a 1971 454 with hyd roller cam, stock pistons and heads.

ctuinstra

As it was explained to me the reason to use the manifold vacuum is that you will have vac advance available at idle, where as using the upper port on the carb will not give you vac advance at idle for better emission control at the expense of idle quality.

Scottd

That much I understand.....but where are you guys hooking it up?

ctuinstra

Sorry bout that. I have a Holly and I connect it to the lower port on the carb which is full manifold vacuum. Maybe your Q-Jet has one also down low. It will be below the throttle plates. If not then T off the manifold vac itself. You might have to get creative if the vac hose sizes differ.

MrJlr

Hmmmmm.....so I can run full manifold vacuum to the vacuum advance on the dizzy?

That would also clean up one more hose going across my engine

ctuinstra

Absolutely! Just make sure to unplug it when checking the idle timing.

Super6

Not running full manifold vacuum to the distributor was an emissions thing. You want full vacuum to your distributor (better idle combustion, smoother cooler running at idle, more throttle response). Ask Lars for his paper on vacuum advance.

Peterbuilt

Regards, Pete.

scottyp99

I may be wrong here, but I get the feeling that you are assuming that the line should be hooked up to the actual intake manifold, which is not the case. Understandable, for something that everybody seems to keep calling "manifold vacuum" There are nipples on the base of the carb that connect to manifold vacuum, (which just means below the throttle blades of the carb) The one that Lars mentioned should be on the passenger side of the carb near the choke, hook it up and drive on, buddy!

Scott

7T1vette

I hook my vacuum advance can to the tube fitting on the passenger side of the carb (tube surrounded by choke mechanism). That is a manifold vacuum fitting.

The headlight vacuum line would best be coming from a fitting on the manifold. Most manifolds have a pipe plug in it somewhere on the surface. Just get an adapter fitting for that pipe thread and insert a T-fitting into it. Then run vacuum line through the line filter to the vacuum reservoir/headlight actuators. Use the other outlet as you need, or cap it off. You can always use that 'extra' vacuum outlet as a place to connect a vacuum gauge when trouble-shooting.

mysticpete

Hi People

Thanks for this interesting post, this maybe why my Vette runs like a woman with an Adam's apple in her throat (Lumpy as!) and then one twitch of the throttle and off she rockets, smooth as.... will have to check this out

7T1vette

mysticpete...

You may also have an accelerator pump problem in your carb. That's the gizmo that gives the intake a big squirt of fuel when you shove the throttle down. No squirt; no 'spurt'....

lars

I did not say that you need to use vacuum directly from the intake manifold - I stated to use a manifold vacuum source on the carb. Why can't you use the port that I suggested on the passenger side of the carb?

Lars

Scottd

I misunderstood you. I have two ports on the passenger side. One near the very base of the carb and points to front of the car ( that is too big for the vac advance line), the other is higher up on the carb body, points straight out to the right fender. Which one do I use?

lars

There is only one port on the passenger side of the carb - use that port, since it is manifold vacuum. It's the easiest hookup for distributor vacuum. The nipple pointing forward at a 45-degree angle on the forward passenger side corner baseplate is ported vacuum intended for use with the EGR system.

Lars

Scottd

Lars, sent you a email. Had no luck with the carb today on install. Let me know what you think. -Scott

v2racing

I agree with Lars on stock or mildly cammed engines should hook the vacuum advance to manifold vacuum. However, large cams with an automatic trans behind the engine often don't work well with vacuum advance hooked to manifold vacuum.

I'll use my own engine as an example. In neutral it idles just under 10 inches of vacuum. I have an adjustable vacuum advance, so I set it to bring in the timing I was looking for at idle. When I put the car in gear, the idle would drop, the vacuum would drop also, it would lose ignition timing and would begin to stall. This was not a new scenario to me, but I wanted to try manifold vacuum first.

To fix this issue, and have enough timing at idle, and have good off idle response, I went to ported vacuum. I limited mechanical advance so that I could get enough initial advance at idle without the vacuum advance and still come out at full advance at the timing my engine liked. I hooked the vacuum advance to ported vacuum on my Holley carb. Using a vacuum gauge, the adjustable vacuum advance and a limiter I made for the total amount of timing the vacuum advance could add, I dialed in best response off idle without getting into detonation and at the same time make the engine happiest with timing at cruise.

The engine would now idle strong in neutral and drop about 150 rpm in gear and idle just as strong. The throttle response was tremendous, and my engine ran cool and got great mileage. Of course I used a digital wide band sensor and a vacuum gauge to dial in the carb too.

If mine had a manual transmission, I probably could have used manifold vacuum.

The OP did not say how big his cam is. It may be his engine is at that point where ported advance and some work to the distributor is needed.

cardo0

Scott maybe u need to test that port. Do u have a vacuum gauge? Connect your vac gauge to the port u choose and verify it has vacuum.

For full manifold vac port i can usually probe it with a steel wire and see where it comes out inside the carb throat. But the pass side port does not go directly enter the carb throat so u may not probe it but u can still measure vac on it.

cardo

scottyp99

An easier way is to just blast some carb cleaner in the port, and observe where it comes out. Please wear eye-protection while doing this, as carb cleaner in your eye, really, really, really hurts. At least, that's what I've heard......

Scott

Peterbuilt

Connect your hose from this port directly to the vacuum advance can.