Quadrajet Replacement
Thread Starter
Burning Brakes
Joined: May 2008
Posts: 948
Likes: 1
From: pensacola fl
Quadrajet Replacement
I'm having trouble with the quadrejet on my 76 with a 4-speed. Seems no matter what I do, I cannot get the secondaries to open. I have read Lars papers but I am not confident enough to do a rebuild. Its kind of funny, I can repair and work on some of the most sophisticated x-ray equipment made, but major automobile work scares me. Any suggestions as to where to get a quadrajet replacement.
Drifting
Joined: Dec 2004
Posts: 1,714
Likes: 42
From: Austin Texas
Melting Slicks
Joined: Mar 2009
Posts: 3,126
Likes: 15
Not an answer to your question but what seems to be the problem with the secondaries? When you go WOT with engine not running but warm, do the lower throttle blades open fully? Can you open the upper air valve manually? There is a lockout on the pass. side of the carb. that may prevent them from opening. Just hate to see you give up on the Qjet without the guys helping you to fix it. Good luck no matter what you decide to do. mike...
Last edited by mds3013; Mar 9, 2011 at 07:38 PM.
Safety Car
Joined: Oct 2005
Posts: 3,570
Likes: 1
From: Algonac Michigan
I know this sounds simple but in watching Lars work on Q-Jets, with the same problem as yours, more times than you might think, simply removing the floor mats will allow them to open fully.
If the problem is more complicated, I agree with what was stated earlier....
Try to fix the carb you have. You have nothing to lose.
The Q-Jet when working properly is a GREAT carb!
Good Luck!
If the problem is more complicated, I agree with what was stated earlier....
Try to fix the carb you have. You have nothing to lose.
The Q-Jet when working properly is a GREAT carb!
Good Luck!
Thread Starter
Burning Brakes
Joined: May 2008
Posts: 948
Likes: 1
From: pensacola fl
I have checked the floor mats. As a matter of fact, with a 4 speed, they always seem to scoot under the pedals. As far as the lockout, it is out of the way. I can see the the throttle blades open when i manually move the throttle linkage but when driving, I know the sound and feel when the secondaires kick in and thats not happening. I just had some aluminum heads installed and it just has not worked right since then.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
The causes of secondaries not opening are limited to the following:
1) the throttle linkage is not actuating far enough when the accelerator pedal is pushed to the floor (NOT the same as opening the throttle with your hand when in the engine compartment).
2) the choke lockout lever (right side on the bottom plate) is not releasing. This prevents the secondaries from opening when you have a cold engine. Even if the engine is warmed up, if not set correctly, you won't be able to open the secondaries.
All of this is clearly described and adjustments are shown in the "Fuel System" section of the Chevrolet Chassis Service Manual. Spend the money to get a copy and "do it yourself". You'll be able to fix your problems and you will have that manual for use on many other 'problems'.
1) the throttle linkage is not actuating far enough when the accelerator pedal is pushed to the floor (NOT the same as opening the throttle with your hand when in the engine compartment).
2) the choke lockout lever (right side on the bottom plate) is not releasing. This prevents the secondaries from opening when you have a cold engine. Even if the engine is warmed up, if not set correctly, you won't be able to open the secondaries.
All of this is clearly described and adjustments are shown in the "Fuel System" section of the Chevrolet Chassis Service Manual. Spend the money to get a copy and "do it yourself". You'll be able to fix your problems and you will have that manual for use on many other 'problems'.
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Race Director
Joined: Oct 2005
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From: Camp Verde AZ
C1 of Year Finalist (performance mods) 2019
2017 C1 of Year Finalist
Holley 6210 4165, 650 CFM spreadbore double pumper.
Best damn Q-Jet replacement ever made. Primary bore are smaller for better fuel economy and part throttle response, and the secondaries are larger to make up for it.
I replaced a Q-Jet on a 396 and a 327 back in the mid 1970's and never looked back.
Doug
Best damn Q-Jet replacement ever made. Primary bore are smaller for better fuel economy and part throttle response, and the secondaries are larger to make up for it.
I replaced a Q-Jet on a 396 and a 327 back in the mid 1970's and never looked back.
Doug
Cruising
Joined: Mar 2011
Posts: 14
Likes: 0
From: Durham North Carolina
I am going with a Holley 4150 vacume secondary 670 CFM to replace my quadrajet too.
I am an engineer and work on diesel engines on ships all the time but was a little reluctant to fool around with a carb. too. But hey we all gotta learn sometime.
My quadrajet was just rebuilt but I'm looking for something with a bit more umph, I'll be taking it off in a week or so and if you find some major problems with yours might be willing to sell my newly rebuilt one.
I am an engineer and work on diesel engines on ships all the time but was a little reluctant to fool around with a carb. too. But hey we all gotta learn sometime.
My quadrajet was just rebuilt but I'm looking for something with a bit more umph, I'll be taking it off in a week or so and if you find some major problems with yours might be willing to sell my newly rebuilt one.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Not trying to be 'picky', here...but exactly how do you get more "OOMPH" from a 670 cfm Holley when you removed a 750 cfm Q-Jet? I'm an engineer, too. And the last time I checked 750 was more than 670. ????
If the performance of your engine/Q-jet is not adequate, I suggest that there is a problem with the linkage/rods/jets or something else in/on the engine. Could the Q-Jet have a problem? Sure. Solution: rebuild it. Just my .02....
Burning Brakes
Joined: Feb 2004
Posts: 924
Likes: 2
From: Kearneysville West Virginia
No question, the Quadrajet is one of the best carburetors out there. You seem like a talented person, so I am confident you can sort this out yourself. Carefully read the manuals and carburetor books suggested. The Quadrajet secondary system is extremely simple and easy to sort out.
Melting Slicks
Joined: Aug 2004
Posts: 2,220
Likes: 6
From: www.Z16.org North/West Pennsylvania
Not trying to be 'picky', here...but exactly how do you get more "OOMPH" from a 670 cfm Holley when you removed a 750 cfm Q-Jet? I'm an engineer, too. And the last time I checked 750 was more than 670. ????
If the performance of your engine/Q-jet is not adequate, I suggest that there is a problem with the linkage/rods/jets or something else in/on the engine. Could the Q-Jet have a problem? Sure. Solution: rebuild it. Just my .02....
Team Owner
Joined: Mar 2002
Posts: 24,160
Likes: 78
Even if you had a 350 running at 100% VE (not actually possible without forced induction), it still is only pumping ~600 CFM at 6000 RPM.
Cruising
Joined: Mar 2011
Posts: 14
Likes: 0
From: Durham North Carolina
The rochester Q-jet flows less trust me!!
4 bbl carbs are rated at 1.5 inches. A 650 carb on a 358 cubic inch engine will seldom see 1.5 inches but a 350 carb on the same engine will frequently see 5 to 8 inches. At 7 inches of vacuum the 350 carb is really flowing about 430-440 cfm in a dry flow.
Vehicle manufacturers often published these numbers from dry flowed tests. This worked very well up through the mid-1960’s, when carburetor comparison tests became popular in car magazines. One carburetor company determined that the results could be skewed by rating their carburetors “dry” (air only), instead of the conventional “wet” (a non-flamable liquid with the density property of gasoline and air mixed). Rating the carburetor dry would add approximately 8 percent to the rating (example – a carburetor rated on the four-barrel rating scale at 500 CFM would now amazingly flow 540 CFM).
Not!!!!
To convert the rating to the 4-barrel wet scale, divide the rating by the square root of 2. 750 CFM divided by 1.414 is 530 CFM
4 bbl carbs are rated at 1.5 inches. A 650 carb on a 358 cubic inch engine will seldom see 1.5 inches but a 350 carb on the same engine will frequently see 5 to 8 inches. At 7 inches of vacuum the 350 carb is really flowing about 430-440 cfm in a dry flow.
Vehicle manufacturers often published these numbers from dry flowed tests. This worked very well up through the mid-1960’s, when carburetor comparison tests became popular in car magazines. One carburetor company determined that the results could be skewed by rating their carburetors “dry” (air only), instead of the conventional “wet” (a non-flamable liquid with the density property of gasoline and air mixed). Rating the carburetor dry would add approximately 8 percent to the rating (example – a carburetor rated on the four-barrel rating scale at 500 CFM would now amazingly flow 540 CFM).
Not!!!!
To convert the rating to the 4-barrel wet scale, divide the rating by the square root of 2. 750 CFM divided by 1.414 is 530 CFM
Last edited by Fletch'sVette; Mar 10, 2011 at 12:41 PM.
Burning Brakes
Joined: May 2008
Posts: 1,116
Likes: 5
From: Middleburg Florida
The rochester Q-jet flows less trust me!!
4 bbl carbs are rated at 1.5 inches. A 650 carb on a 358 cubic inch engine will seldom see 1.5 inches but a 350 carb on the same engine will frequently see 5 to 8 inches. At 7 inches of vacuum the 350 carb is really flowing about 430-440 cfm in a dry flow.
Vehicle manufacturers often published these numbers from dry flowed tests. This worked very well up through the mid-1960’s, when carburetor comparison tests became popular in car magazines. One carburetor company determined that the results could be skewed by rating their carburetors “dry” (air only), instead of the conventional “wet” (a non-flamable liquid with the density property of gasoline and air mixed). Rating the carburetor dry would add approximately 8 percent to the rating (example – a carburetor rated on the four-barrel rating scale at 500 CFM would now amazingly flow 540 CFM).
Not!!!!
To convert the rating to the 4-barrel wet scale, divide the rating by the square root of 2. 750 CFM divided by 1.414 is 530 CFM
4 bbl carbs are rated at 1.5 inches. A 650 carb on a 358 cubic inch engine will seldom see 1.5 inches but a 350 carb on the same engine will frequently see 5 to 8 inches. At 7 inches of vacuum the 350 carb is really flowing about 430-440 cfm in a dry flow.
Vehicle manufacturers often published these numbers from dry flowed tests. This worked very well up through the mid-1960’s, when carburetor comparison tests became popular in car magazines. One carburetor company determined that the results could be skewed by rating their carburetors “dry” (air only), instead of the conventional “wet” (a non-flamable liquid with the density property of gasoline and air mixed). Rating the carburetor dry would add approximately 8 percent to the rating (example – a carburetor rated on the four-barrel rating scale at 500 CFM would now amazingly flow 540 CFM).
Not!!!!
To convert the rating to the 4-barrel wet scale, divide the rating by the square root of 2. 750 CFM divided by 1.414 is 530 CFM
Now I'm curious. I don't understand, 1.5 inches of what?
Cruising
Joined: Mar 2011
Posts: 14
Likes: 0
From: Durham North Carolina
The standard vacuum gauge reads in inhg, which stands for inches of mercury, kind of the opposite of PSI.
The best way to explain it is using a tube style anemometer. So if I took a u shaped anemometer tube and had filled it with mercury and plugged one end into my carb vacuum port and the other end was left open to the atmosphere it suck the carb end to 1.5 inches higher then the atmosphere side. Vacuum is actually the absence of pressure in a system when compared to the atmosphere.
The best way to explain it is using a tube style anemometer. So if I took a u shaped anemometer tube and had filled it with mercury and plugged one end into my carb vacuum port and the other end was left open to the atmosphere it suck the carb end to 1.5 inches higher then the atmosphere side. Vacuum is actually the absence of pressure in a system when compared to the atmosphere.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
There's no doubt that a 'normal' 350 cid engine will not need [or use] all of the available capacity of a 750 cfm Q-Jet. But, that has nothing to do with a response that indicates a 670 cfm Holley will produce "more" power than the Q-Jet. The carb 'flow' is only as good as the intake and heads will allow. However, that isn't the contention here. I'm questioning the validity of a 670cfm Holley having more 'oomph' than a Q-Jet.....nothing more.
Cruising
Joined: Mar 2011
Posts: 14
Likes: 0
From: Durham North Carolina
So essentially its a moot point yes, We'll just leave it at some prefer q-jets and the way they have been tuned and others prefer Holley and they way they tune. No doubt that both can get the job done, I prefer the easy swap out spring on the secondary's for the Holley and felt that the OP might want some less technical tuning details for their carb.