try this I think at the cost of a non-matching rebuild. These might be better.http://store.summitracing.com/egnsea...alse&N=700+115

 

Actually, the Q-Jet secondaries are "airflow" activated. The airflow required by the engine causes the secondaries to open and fuel is regulated accordingly. (no vacuum required)

 

Stay away says the all knowing LARS!!! and

 

Hate to dissagree, but the air valve is controlled by vacuum

 

Sly_vette....

Perhaps you should do some reading-up on Q-jet operation:

"The secondary side of the Quadrajet has two large (2-1/4") bores. These, added to the primary, give enough air capacity to meet most engine requirements. The air valve is used in the secondary side for metering control and supplements the primary bores to meet air and fuel mixture requirements of the engine.

The secondary air valve mechanically operates tapered metering rods which move in orifice plates, thereby
controlling fuel flow from the secondary nozzles in direct proportion to air flowing through the secondary bores."

Maybe you should know a little more about the things you try to make fun of.

 

You didn't mention however, that what allows everything you describe to operate is vacuum, or the lack of it.

The air valve is held shut by the vacuum controlled diaphragm mounted on the choke linkage.(plastic or metal can).As the diaphragm is draw in by vacuum,(idle),the rod connected to it and the air valve, is drawn along with it, locking the air valve closed. You can test this by trying to push the air valve open when idling.

Opening the throttle decreases vacuum, releases the diaphragm and air valve rod, and allows exactly what you say to happen. Makes for a very smooth transition.

 

So a vacuum-tripped, mechanical release of the secondary mechanism is how you define the secondaries being "operated" by vacuum? Interesting way to 'save face'....

 

Thank you.(I think). Save face??? If you read my reply to your," no vacuum required "comment, I said that vacuum controls the AIR VALVE.

Guess how all that "air flow" you talk about gets generated. Not to mention the air valve lifts the needles to start the fuel flow as well. What controls your air flow? What controls the air valve? Good old vacuum.

Dont think you need it? Just block it off. See how that mechanical secondary works for you.

What are we arguing this for? Its not that important or hard to understand. If you think there's no vacuum, that's fine.

 

I replied to "Sly-vette". You chose to stand up for him and "go to bat". Your choice...not mine. He did his cute little 'mocking' thing and you went along for the ride. Peas in a pod.....

Maybe you and your buddy ought to treat this Forum as a place for mutual support for fellow C3 owners...rather than to "attempt" some ignorant mockery.

 

See your choosing to get away from the tech/ performance of this forum. I apologize to anyone reading this nonsense, but need to answer, and this will be the last word from me.

I answer for my own comments. I certainly didn't mock you. Just corrected you.

As far as giving support and info on this, or any of the forums, I won't even bother to answer that. All one needs to do is check this thread to see who's comments support and give info. The only info you gave is wrong ,and all your other following comments add nothing.

If in correcting you I was insensitive to your inputting info without knowing the subject, I'm sorry. If You DO know the subject, and don't like to be corrected, then shame on you.

 

I know one thing...there are only two things that will move air. Either a vacuum or pressure...and what operates the secondaries on a Q-Jet ain't pressure!!!

I think all we have here fellas is a simple mis-understanding...If I stepped on someones toes I heartily apologize!!!
Mark

 

I bought a Q-jet from Jegs Racing that was built by Jet Performance. It's a 750, electric choke, level II (modified for better off-idle response), vacuum secondaries. The secpmdaries kick in immediately when I put my foot in it. It has fully adjustable idle mixture. I have it on 1979 slightly modified L-82. The carb works great and has no hood issues. I like the electric choke also.

 

how is it doing now ???

 

Lots of misunderstandings on the operation of the secondaries on a Q-Jet carb.. pretty common.

To understand the operation of the Q-Jet, first you have to understand what a “vacuum secondary” carb is and how it functions.

A “vacuum secondary” carb is a carb whose secondary throttle blades are opened by the force created by venturi vacuum in the primary side of the carb. The vacuum created in the venturi of a carb is directly proportional to the mass flow of air passing through the venturi. This venturi vacuum is completely independent of manifold vacuum, which is non-existent at wide open throttle (WOT). A vacuum secondary carb has a little hole drilled right in the venturi on the primary side, and this venturi vacuum is fed to a spring loaded diaphragm attached to the secondary throttle shaft. Once airflow on the primary side approaches the maximum flow capability of the primary venturi, the vacuum will be high enough to overcome the diaphragm’s spring pressure, and the secondary throttle is opened by the primary venturi vacuum. This is a vacuum secondary carb.

The Q-Jet does not have any vacuum holes drilled in the primary venturi, and there is no vacuum diaphragm attached to the secondary throttle shaft. The Q-Jet is not a vacuum secondary carb – it is mechanical with a secondary airvalve control.

But vacuum sucks the airvalve open, and the airvalve is connected to a vacuum diaphragm, so it’s vacuum operated, right?

Not really. Imagine this: Take a spring-loaded screen door and set it up right out in your front yard. As the wind starts blowing, the door gets pushed open. The harder the wind blows, the more the door gets pushed open. Do you have a vacuum on one side of your front yard sucking the door open..? Of course not – the pressure is the same all over your yard. The force opening the door is the mass flow of air pushing the door open. There may be a low pressure area in Texas (since Texas sucks…ha!) that is causing the air to move, but Texas is not “sucking” the door open – mass air flow is pushing it open, and the door is responding to the actual total mass air flow being pushed through it. The Q-Jet operates the same way: At WOT, there is no vacuum in the manifold – the manifold is very close to atmospheric pressure (a correctly-sized carb will cause the manifold vacuum at WOT to be at about 0.5” Hg, which is nothing). So the force opening the airvalve is the same as the wind pushing your yard-mounted screen door open: It’s mass flow pushing it open. This is not a vacuum operated carb. There is no vacuum in the manifold at WOT, but there is plenty of mass airflow.

The diaphragm on the side of the Q-Jet “controlling” the secondary airvalve is actually the choke pulloff. It is also connected to the airvalve to hold it firmly closed when manifold vacuum is high. When the engine is placed in a power condition (WOT or low manifold vacuum), the diaphragm relaxes at a controlled rate to prevent excessively sudden opening of the airvalve: The longer the airvalve is delayed in its opening, the bigger “fuel shot” the secondaries get upon opening, thus preventing a secondary tip-in stumble. The pulloff merely allows a controlled opening rate of the valve, and is not a vacuum-operated control of the secondary throttle in any way.

Thus the Q-Jet is not a vacuum secondary carb. It is an airvalve-controlled mechanical secondary carb. The airvalve is not operated by vacuum – it is operated by mass flow. The airvalve’s opening rate is controlled by the loss of vacuum signal – not by the creation of any vacuum.

For some interesting reading on the 3 different types of engine vacuum, feel free to drop me an e-mail request for my “Engine Vacuum Explained” tech paper:

V8FastCars@msn.com

Lars

 

Temvette72

 

I'm in the same situation, I would like to replace my Q-Jet with a Speed Demon and am wondering what air cleaner will fit under my stock 69 SBC hood?

 

On a stock manifold and especially if you have stock internals I think the Q-jet is a better carb and the Q-jet has proven reliable and when tuned properly just as potent as a holley for more hp than most street rides put out.

I had a 650 holley spreadbore on my 69 400 pontiac and it ran well but it wasn't better than the Q-jet IMO.

A lot of Pontiac guys make huge hp with Q-jets on big inch ponchos.