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Old 10-05-2011, 06:33 PM
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Rochester Quadrajet Plumbing ???

I picked up a new rebuilt quad from Summit and of course the directions are incredibly basic. When I say basic, it starts with put the e brake on. There's nothing in this pamphlet regarding plumbing any of the 6-7 vacuum "or whatever they are" ports that are on this carburetor. If anybody could steer me to a link or a book that will tell me what to do with all these outlets, I'd appreciate it. I looked in the Wiki and knowledge base, but really couldn't find what I was looking for. Thanks for your help!

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Old 10-05-2011, 06:37 PM
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I probably should've mentioned that this is all on a truck that was originally a diesel. No emissions. I will be running a vac advance distributor, and a PCV valve, but that's about it.
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Old 10-05-2011, 08:46 PM
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Quote:
Originally Posted by frodin1
I probably should've mentioned that this is all on a truck that was originally a diesel. No emissions. I will be running a vac advance distributor, and a PCV valve, but that's about it.
You will want to identify a ported vacuum source and a manifold vacuum source or two. Then decide either by prior knowledge or by trial and retrial which type of vacuum (ported or manifold) you want to use for the vacuum advance.

The PCV is ported into the front large port, the rear port (usually has a fitting threaded into the throttle body) is used for the power brakes unless you use a port off of the intake runner for this. Once everything that needs vacuum is plumbed, plug any remaining vacuum ports w/good quality caps- the parts store "HELP" aisle caps are junk for the most part, try to find either OEM plugs or silicone plugs.

If you have an automatic that uses a vacuum modulator, this also uses a manifold vacuum source. Often the factory used a fitting that was into the intake manifold for this but you can use a port on the carb.

There is one vacuum port on the carb that was used for the EGR valve. It is not suitable for use as a manifold or ported vacuum source. The problem is, I cannot say exactly which port this will be on all carbs, but it is often the port on the front, passenger side in the throttle body, nearest to the idle mixture screw.

If you don't have a port there, or you want to check it (as I would), one way to tell is to use a vacuum gauge- which you should be using anyway, come time to set the idle mixture screws, etc. The EGR port will not have as much vacuum at idle as a manifold port- but it will have enough vacuum to easily register on the gauge, unlike a ported vacuum port that will show less or no vacuum at idle. Because of where the EGR port picks up its vacuum signal (the venturi) it will tent to show more vacuum on the gauge as the engine speed increases. The EGR port will sometimes be a larger diameter than the other vacuum ports- test those first.

Yet another way, though a crude one, to ID the EGR port, is to blow smoke through the vacuum ports until you find the one that has smoke coming from an orifice that's in the venturi area of the carb. You'll need a cigarette and a length of vacuum hose to pull this off.
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Old 10-05-2011, 10:25 PM
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It should be noted that Q-jet carbs prior to the introduction of the vacuum operated EGR system obviously will not have an EGR vacuum port. These carbs having an EGR vacuum port are mainly going to be from about ’71 or so and newer.

Carb numbers:

• 1971- 7041xxx
• 1972- 7042xxx
• 1973- 7043xxx
• 1974- 7044xxx
• 1975- 7045xxx
• 1976-’79- 1705xxxx
• 1980-up- 1708xxxx


Also:

“Because of where the EGR port picks up its vacuum signal (the venturi) it will tent to show more vacuum on the gauge as the engine speed increases.”, should read:

“Because of where the EGR port picks up its vacuum signal (the venturi) it will tend to show more vacuum on the gauge as the engine speed/throttle angle increases."
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Old 10-06-2011, 02:48 AM
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I'd open it up and tune it with a Quadrajet kit from Cliff... He sells soft parts that are methanol friendly...

http://www.cliffshighperformance.com/
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Old 10-06-2011, 09:43 AM
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First thing, the brakes I have are hydraulically boosted, off the power steering. So I don't need that port. Now, should I be running an EGR valve? Because, I currently do not have one? There's is a small vacuum line on the carb already, but I'm assuming it's for the secondaries? I live in an area that does not require emissions on vehicles. so I don't know if this is something that would be required for that reason or not. As far as the carb, These are supposedly wet tested and run. Me, not being much of a carb guy, like the idea of being able to fire this thing up, without having to mess with it. Except of course the fine tuning that's usually required. Basically it looks like I need the PCV, Trans, and vac advance. Everything else will probably get plugged. Unless of course I have to have an EGR valve?
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Old 10-06-2011, 10:05 AM
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Quote:
Originally Posted by frodin1
Now, should I be running an EGR valve? Because, I currently do not have one?
Don't use an EGR in this case. Unless you had to comply w/emissions and the engine was set to all OEM calibrations it will not help anything, so forget about it.

The main reason I went into all the details of how to recognise the EGR port is so you wouldn't inadvertently use it for the vacuum advance, for example. Doing so could damage the engine.

Quote:
There's is a small vacuum line on the carb already, but I'm assuming it's for the secondaries?
Most likely that is the short vacuum line from the passenger side front corner of the carb that curves around to the choke pull off. Leave it as-is, and yes, the pull off also modulates the opening of the secondary air valve.

Quote:
Basically it looks like I need the PCV, Trans, and vac advance. Everything else will probably get plugged. Unless of course I have to have an EGR valve?
That looks right to me. As I mentioned earlier, select the source for the vacuum advance based on what you know works, or by trying it on one then the other (ported or manifold).

BTW, "ported" vacuum is when the vacuum port has no (or very little) vacuum at idle, but has vacuum once the throttle blades are open- like as you drive down the road.

"Manifold" vacuum is present both at idle and while the throttle blades are open. For many performance engines, using manifold vacuum works well. It tends to smooth the idle, it will create more vacuum at idle and will help the engine to run cooler at idle.
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Old 10-06-2011, 10:21 AM
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Alright, cool. Thanks for the info!
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Old 10-06-2011, 11:00 AM
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You're welcome. And thank you. I had been meaning to do a page on ID'ing vacuum ports. This thread got me to go ahead and do it!
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Old 11-24-2011, 10:30 AM
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I can't seem to figure out what the small line is coming out of the rear/top of the bell housing on the TH400- 1/4" steel? I can't remember if I had a line going to this or not? Is this a vent? HELP anybody?
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Old 11-24-2011, 10:44 AM
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Quote:
Originally Posted by frodin1
I can't seem to figure out what the small line is coming out of the rear/top of the bell housing on the TH400- 1/4" steel? I can't remember if I had a line going to this or not? Is this a vent? HELP anybody?
It is the case vent, does not need to be hooked to anything.
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