lars

Lots of questions and misunderstandings on the operation of the secondaries on a Q-Jet carb, so I thought I'd put together a quick explanation of the system. Contrary to popular belief, the Q-Jet, typically rated at about 750 cfm, is not a vacuum secondary carb - it's mechanical with some interesting controls. Back in the 70s, I taught classes at the GM Training Center on the Q-Jet, and this question frequently came up. The following is the explanation as taught by Rochester at the GM Training Centers in the early 70s:

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 (not manifold 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 force of 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 a mechanical carb with a secondary airvalve control: The secondary throttle blades are mechanically opened by progressive linkage from the primary throttle lever.

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… ) 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 not enough "suction" to "suck" the airvalve open). 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. Think of the pulloff as the pneumatic cylinder damper found on most screen doors: The screen door cylinder does not open the door - it just controls and dampens its opening rate. Same concept on the carb.

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 dampened by the loss of vacuum signal to the pulloff – not by the creation of any vacuum.

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

V8FastCars@msn.com

Lars

RATT7

Thanks for the tech session, I will make a copy for my car binder. Good stuff to know !!

Jim Shea

Lars,
Good to see you posting again. Hope things are going well out there.

You may note that the Danbury Mint has just released a 1/24 die cast, 1975, 7Up green, L82, 4 speed, tan custom interior, convertible. It even has a luggage rack on the back. I could hardly ask for a more exact duplicate to mine.

Jim

Matt Gruber

my tires are NOT made in china
.
made in Korea
.
the Q-jet has earned whatever reputation it has.
Describing it perfectly won't change it.
.
Lars, without guys like u to fix it, it would be RIP (resting in pieces)

GreenDream

Thanks Lars...without you, we would all have to go buy C5s or forgive the thought, C4s.

BarryK

say what you will but the QJet on my C3 is a better operating carb with less problems than the Holley on my C2.

427Hotrod

Hey,,,what's that about Texas??????

Don't make me come back up there!


Thanks Lars...good stuff as always!


JIM

NuclearBob

Nice to see you are back!! Hope all is well!

sly vette

Lars,
Sometimes you make my head hurt!!!!

RIJVETTES

Thanks Lars. Hope you're feeling well. I recently rebuilt the engine- but kept the Q Jet which is still working excellent.

DaveR-IL78

Lars,
Thanks for that explanation. I'll read it over...and over...and over.
Dave

Peterbuilt

Hi lars, Great to have you back! You are the "Go to Man" on carb's.
Sooner or later I'll have you do mine.
Again, Welcome Back, PG.

sxr6

Good to see you back Lars

Got a quick question...is there a proper way to adjust the choke on a quadjet?? I'm refering to the older intake manifold heat coil type as opposed to the newer electric dial-a-choke

lars

Yes, there is. For step-by-step setup on the Q-Jet choke systems (both types) drop me an e-mail request for my Q-Jet Choke Setup Paper:
V8FastCars@msn.com
Lars

lars

Hi Jim!
I'm trying to give you some incentive to come back up here, even if it's to kick some butt...! The Texas State Flag you left here is still proudly displayed in the workshop, regardless of the suction conditions in Texas...

Little Mouse

Ive lived in texas since 78 it does not suck it blows much
like the description of the help the mechanical secondaries get.
80 percent of texas is flat as a board the wind blows hard here Think of texas as a big overgrown Kansas without the colder winters.

Stormin123

Great Post Lars! Too many people have a negative attitude about Q-Jets. I still rebuild a few of these every year. Everyone thinks they are a pain but I love em.

Stormin

sweethence

Bah i say, nothing against Lars, I hear he does fine work, But I can rebuild and run my own Qjets and have done so for the last three cars (two camaros and a vette). Its not rocket science and they are well documented devices. I use mine (the vette that is) as a DD and it runs like a raped ape up to about 5500 when i run out of cam.

Nothign wrong at all with the Qjet, I rebuilt my first one with the book thats out there and never looked back

Aggitated Monkey

Wow! That explains all the WIND we get here in Kansas; does that mean Oklahoma is an orifice or I mean Venturi

Thanks Lars, That was excellant and gives some true understanding of the Q-Jet.

I know that the Q-Jet gets a bad rapp, however I see it as being very dependable and many great running GM cars (lot of Cadillacs) from the '60's and 70's are still running smoothly with a Q-Jet.

SLVRSHRK

Lars,

Great reading a always, but I have a question.

I went and bought a screen door to mount on the car, but with all the curves I had a hard time getting it mounted. But I tell you, once I got the door on the car, it definitely flew open when I punched it Any suggestions on the type of damper I should put in to make sure it opens gradually?

Say what you will about the quadrajet, but it really does adapt well to changing situations. I made no jetting changes to my stock 78 quad when I rebuilt the motor (better heads, cam, manifolds, dual exhaust, etc.) and the LM-1 shows that it survived the change and really performs well.

No need to get rid of these.