Hi all,

My friend has a 1971 Corvette with a 4 speed and it needs a carburetor rebuild. I have done some research and found he has a carburetor for an automatic on his car. The part # is 7042202 and built by Carter.

I am told two different things from two different mechanics. One says to replace it with the correct manual trans carb part # 7041213 because the one on it now is for an auto with vacuum secondary and the car needs a mechanical secondary since it's manual. The other mechanic says to rebuild the one for the automatic that's currently on the car because the mechanical secondary one will dump too much fuel and will never run right.

What do you guys think we should be doing here. Need opinions.

Thanks!

 

Get rid of the mechanics you're talking to as they are completely clueless. All Q-Jets are mechanical secondary carbs with an airvalve control - no difference at all between secondary systems on automatic or manual carbs. There is no such thing as a vacuum secondary Q-Jet. There is no difference at all between the manual and automatic carb other than that the automatic carb may have provision on the throttle lever for a turbo 350 kickdown cable attach point. Use what you have - it will run absolutely identical to the manual trans carb. Don't let either of those mechanics touch the carb - they're idiots:

"The mechanic says the mechanical secondary one will dump too much fuel and will never run right."

That's about the dumbest thing I've ever heard... Good luck with the project - hope you can find someone competent to assist you!

Lars

 

Listen to Lars

 

I'll go one better..Call Lars and get on his waiting list.

Brian

 

No brainer

 

No waiting list... I have no current backlog, and I'm keeping up with all work coming in.

Lars

 

Joe -
Here is a write-up and explanation I did a while back about Q-Jet secondary systems. I re-post this every few years as people forget about it... Read this, and you will know 10 times as much about Q-Jets as the hack "mechanics" you're dealing with. Every Q-Jet secondary system works exactly the same - there is no difference between manual and automatic carbs.


by Lars Grimsrud
Tuned by Lars Corvettes & Musclecars
Denver - USA

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

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 into 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 a mechanical carb with a secondary airvalve control. There is hard, mechanical linkage connecting the primary throttles to the secondary throttles, and the secondary throttle plates are mechanically opened at a progressive rate by the throttle linkage. Purely mechanical.

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 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. Think of the pulloff as the damper cylinder on the screen door: The damper cylinder does not open the screen door – it merely controls and dampens its opening rate.

Thus the Q-Jet is not a vacuum secondary carb. It is an airvalve-controlled mechanical secondary carb with a damper. The airvalve is not operated by vacuum – it is operated by mass flow. The airvalve’s opening rate is controlled and dampened 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.

 

lars you are the man. I don't know how many youtube videos I have watched on rebuilding a quadrajet carb, and they all say they have vacuum secondarys. Just goes to show anyone can make a video, even if they don't know what they are talking about.
Thanks for all your help on the forum
Lawrence

 

Lars RULES!

 

Awesome everyone, thank you! Lars, thank you. We originally wanted to go with rebuilding the one on the car because it was the least expensive and now we will just go ahead and have that done. I appreciate the article!

 

Just be careful about who you have do the work. I only know about 3 people in the country that will do it right, and it's not either one of the two "mechanics" you've been talking to....

Lars

 

Haha will do

 

Lars is so good that my rebuilt carb works upside down

think about it, i'm from ɹǝpunuʍop!

 

Dear CamJag...do yourself a favor and have Lars do your carb work....you will never regret it. Contact him at... V8FastCars@msn.com

 

Hi camjag, i see you are a nutmeg'er too...

Lars, was very helpful when i rebuilt my Q-jet on my 69 L-46.
If it's a stock rebuild you should be able to handle it, or farm it out to someone with references. I got a quote of $300 to rebuild so i did myself.

Got a rebuild kit from Cliff Ruggles and got his book.

Good Luck.
-ALF out...

 

Same for me on same car, carb was unmolested which was key, with tech support from Lars it was pretty basic. Good luck

 

Hi Lars,

Could you explain the turbo 350 kickdown cable that you mentioned above? Did any C3 (specifically '72) have a kickdown cable or was the attach point simply there for other vehicles that used the same carb?

Thanks as always for your expertise,

Steve

 

Where's my book? Went to a GM quadrajet tuning class back in the late 70s. I liked holleys until I went to that class.
They taught about removing the lead seals covering other adjustments.

Don't remember any details 😔

 

Two year old thread, guys.....

 

Still good info and applies to today also