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FYI - Rochester carbs

Started by fishnjim, April 12, 2020, 09:36:28 AM

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fishnjim

https://www.rodauthority.com/news/throwback-thursday-rebuilding-the-quadrajet-carburetor/

This contains a link to orginal article.   Maybe helpful to some, maybe not for others.   Lots of forum questions come up on these.   When it was all we had, we got pretty good at maintaining them.   But passe tech in today's electro-mechanical world.   

cadillacmike68

Sure, modern multi port and direct injection systems controlled by sophisticated on board computers that closely monitor and control fuel flow and delivery have provided modern cars with greater power, lower emissions and greater fuel economy ALL AT THE SAME TIME, but back in the late 60s to early 70s, the Rochester QuadraJet was a fantastic carburetor.

The small efficient primaries gave good idle and slow speed cruising, while the huge secondaries provided ample part and full throttle response. They were rated up to 800 CFM, as much as as any holley or edelbrock of that time.
Regards,
"Cadillac" Mike

Jon S

The Rochester Quadra jet was introduced in 1965 and was so successful that Ford copied the basic design with the introduction of their Autolite 4300 in 1967.
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

TJ Hopland

One thing for sure none of the other carbs sounded like the Qjet.  That is one thing I miss about my EFI.   

I think they even ended up on a few Fords.  I seem to recall a Quadrajet section in a 1966 shop manual.  It didn't apply to my car so I never looked close at it.  I assume it was a California or altitude option for some model that year perhaps a full size car with the 390 was it?

Wasn't there also a couple years and models that if equipped with the Qjet didn't have to have a cat when pretty much everything else but a couple Honda's had to have em?

GM trucks used them through 86.  Many of the remaining RWD cars used them through 87.  88-90 as far as I know it was just the Brougham and full size wagons.   I also seem to recollect running into an Isuzu SUV possibly into the early 90's that had one.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

67_Eldo

The QJ on my '67 Eldorado was rebuilt (at great expense) a few years ago, but the idle was always a bit lumpy. I finally got very serious about chasing down the last of the lumps a few weeks ago -- after years of chasing vacuum leaks -- and determined that my carb had not been rebuilt well. If you cleaned it with carb cleaner and then looked at it again a few days later, you'd see that practically every seam in the body was oozing just a bit of gas.

My first thought was to rebuild (again) myself. But after reading numerous articles describing how QJ bodies warp and do weird things, coupled with my long-ago unsuccessful attempts to meaningfully improve QJ performance using the $30 rebuild kits, I decided to purchase a rebuilt carb from a pro. So I ordered a unit from CarburetorCenter.com.

A few days later, a gleaming QJ arrived. Its vacuum-connection locations differed slightly from my "original" for a couple of reasons. First, the number stamped on the "original" carb indicated that it came from 1969, not 1967. That's OK because the '69 carb is recommended as an upgrade to the '67 carbs anyway. Also, this carb was actually made by Carter under license from Rochester. But it bolted up to the manifold and linkage exactly as did my previous carb, so all was well.

When I pulled my old carb, I noticed that there was a gasket between the carb and the stainless-steel shim as well as another gasket between the shim and the manifold. I called Carburetor Center and asked them if they should have sent me a second gasket (one came with the carb). They said that the carb could be mounted with either one gasket or two. I went with the layout specified in the manual, with the shim right up against the bottom of the carb (which had been machined flat), then the gasket, then the manifold.

I torqued the bolts and screws down, hooked up everything else -- running the manifold vacuum straight to the distributor instead of through the Eldorado's thermostatic vacuum-advance-control switch mounted on the radiator -- and fired it up.

With no adjustments at all, the 429 finally idled as smooth as glass. There was a bit of a gas dripping from the fitting, but a bit of Teflon tape fixed that.

I have a Bosch O2 sensor bunged into my exhaust manifold. The ratios are running between 13.5 and the golden 14.4. It cost $400 (minus a $95 core fee when I send the original back to them), but at the moment, I'm a happy camper, glad that I didn't attempt the rebuild myself.

EAM 17806

Would you all say the 1975-78 Rochester carbs were the most advanced insofar as efficient and reliability is concerned?   EAM
Ev Marabian

1976 Cadillac Coupe DeVille, 1989 Chevrolet Caprice Classic, 1990 Pontiac Bonneville and 1996 Buick Skylark

Jon S

I would say the 1981’s were the most advanced due to the fact that they were tied to the computer command control with a electronic sensor.
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

79 Eldorado

There's a very good "HPBooks" book called "Rochester Carburetors" by Doug Roe. I bought that prior to rebuilding my first QJet and it certainly helped. It also covers more than just the QJet but guess which one is pictured on the cover?

HPBooks-014 ISBN 0-89586-301-4
Mine is the 1986 3rd printing, revised edition
Library of Congress Catalog Number 86-81204

Regarding "most advanced". Adding electronics to it may make it more advanced but I don't think that made them more reliable. Personally I think I would stick to one of the ones prior to with electronic integration. The book I'm referring to mentions that the "microprocessor-controlled Q-jets are post 1981.

Scott
PS: I wonder if Doug Roe is related to our Bruce Roe? The level of information in the book almost seems to imply a family relation ;)

TJ Hopland

Quote from: EAM 17806 on April 14, 2020, 01:54:25 PM
Would you all say the 1975-78 Rochester carbs were the most advanced insofar as efficient and reliability is concerned?   EAM
I too have read that that was the best years especially for Cadillac that used 800's.   I have also heard that the computer ones are fine and can be easily converted to non computer units or you can do home brew stuff to use some of the computer bits to your advantage. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

J. Skelly

#9
Ford used the Quadrajet on some of the '70 429 engines used on the Torino (probably Cyclone and other FoMoCo models, too).  It was a 700cfm unit and was better able to pass emissions than the other carbs Ford used.  Read the article in the March 2020 issue of Hemmings Muscle Machines.

The Quadrajet is a vastly underrated carb.  It provides excellent throttle response, meets emission requirements, and gets good mileage (relatively speaking) when driven carefully on two barrels.  As with any carb, if it is gummed up or needs rebuilding, it will run poorly.  It takes patience to get it running satisfactorily on our old cars with the fuel we have today.  It took me several days of driving home from work and then making fine adjustments to get it operating the way I wanted it to on my '77 Eldorado when I first got it.       
Jim Skelly, CLC #15958
1968 Eldorado
1977 Eldorado Biarritz
1971 Eldorado (RIP)

MaR

Q-jets get a bad rap mostly because people that don't know what they are doing mess with them. They think they can "improve" on the design by removing parts they think are emissions controls when they are actually parts of the system that meters fuel, air, vacuum or some combination of those. When they run worse, they throw a generic aftermarket carb on it which would run better but probably not quite as well as a properly tuned factory Q-jet would have.

TJ Hopland

I think another problem is a 300-350 in a Chev or Ford type of car that's owner things its a race car is quite a different animal to tune than the 390-500 found in the likes of a Cadillac or Lincoln.   The types of mods that work on a mid to high revving engine in a car with fairly tall gears is a lot different than something like a Cadillac that may almost never see 4k rpm. 

There were some way more complex carbs than the Qjets but I don't think they were as commonly used likely because they were non reliable or didn't do what they were supposed to do. 

I used to see it a lot in my industry when it was still mechanical with 2 machines that both did the same thing but with significantly different designs.    One design wasn't that complex and had a lot of places to make adjustments and was pretty easy to work on.   Another design was way more complex, didn't have many adjustments, and really took some practice and skill to work on.   

It was a darn good thing the simple one was easy to work on because we had to work on them and adjust them all the time.  Many times I think it was just because the adjustments were not staying in place either from vibration or what we used to call 'screwdriver drift' which was when the operators would adjust things they were not supposed to for one reason or another.  The adjustable one also had a lot more sealing surfaces so also tended to leak the most which meant if the operators didn't maintain the lubricant levels they tended to self destruct pretty often too.   The other ones once you understood them for the most part just did their thing.  I miss those days.  Now I stand there with a computer hooked up connected to the manufacturer where they ask me if I tried turning it off then on again.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

cadillacmike68

Quote from: 79 Eldorado on April 14, 2020, 03:30:23 PM
There's a very good "HPBooks" book called "Rochester Carburetors" by Doug Roe. I bought that prior to rebuilding my first QJet and it certainly helped. It also covers more than just the QJet but guess which one is pictured on the cover?

HPBooks-014 ISBN 0-89586-301-4
Mine is the 1986 3rd printing, revised edition
Library of Congress Catalog Number 86-81204

Regarding "most advanced". Adding electronics to it may make it more advanced but I don't think that made them more reliable. Personally I think I would stick to one of the ones prior to with electronic integration. The book I'm referring to mentions that the "microprocessor-controlled Q-jets are post 1981.

Scott
PS: I wonder if Doug Roe is related to our Bruce Roe? The level of information in the book almost seems to imply a family relation ;)

My edition has a Pontiac carb on the cover. 1981 revised ed. looks like 12th printing. same ISBN and LC #s

The inside front page has a line drawing of an electric choke Cadillac carb.

there is also a book by Cliff Ruggles on QJets: How to Rebuild and Modify Rochester QuadraJet Carburetors
Regards,
"Cadillac" Mike