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Old 08-04-2007, 08:30 PM   #1
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4 barrel quadrajet rochester

I have a 4 barrel quadrajet rochester, I was wondering if anyone could tell me how many turns the mixture adjustment screws needs to be turned from the base set (both screws all the way in).

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Old 08-04-2007, 08:50 PM   #2
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Man, those things are a Black Art. That is the single item on my SOB RV that I farmed out.

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Old 08-04-2007, 08:54 PM   #3
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A lot depends on the displacement, but for a rough start i would use 2.5 turns out. Go from there, and good luck! Use a vacuum gauge and tach from there.
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Old 08-04-2007, 09:05 PM   #4
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I switched both of mine (2 different 455's) with Holley Spreadbores. What a difference! Glad I got rid of the Rochesters.

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Old 08-04-2007, 09:15 PM   #5
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I'd use about two turns out as a starting point. What I do is turn them in (clockwise) in quarter turn increments until I detect popping in exhaust (lean idle misfire). I turn them out until this misfire just goes away. A lean idle misfire will increase unburned HC; a rich mixture from turning the screws out too much will increase both HC and CO. Some emission carburetors have a rather limited effect of these adjustments.

If you're subject to emission checks, you may need to have this adjusted with instrumentation.
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Old 08-04-2007, 09:28 PM   #6
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Long Time Ago

Hi, it's been a long time since I have had my hands on one of those. [about 35 years ago] The rebuild kits used to have a rubber biscuit that had to be installed inside between the base and main body to stop internal leakage; And the throttle shafts used to get sloppy and cause erratic idles. The throttle shaft bores had to be drilled and rebushed. These concerns have to be repaired before any adjustments will work effectively.
Other than that, you need to do what markdoane said [post #3] or change to Holley spread bores like Neil did. [post #4]

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Old 08-04-2007, 09:34 PM   #7
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I start at 1.5 turns out from closed. After each adjustment, blip the throttle once to clean it out, let it idle back down, then adjust to stumble and back out half a turn. Do this from side to side a few times (I usually work three times) adjusting the idle as necessary. If a vacuum gauge is available and a tach use to get the highest vacuum you can. I've been playing with Quads for a long time and have found that this works well. If emissions testing is an issue lean it out another half to three quarters of a turn back in raising the idle at the same time to keep it on the lean side, but change it back to where it was as soon as you can as it will be too lean this way for your engines needs. Also, this does not always work with emissions testing, but it has worked fine for me over the past many years before I switched carb brand.

As a note, after years of Quads and Holley's I bought an Edelbrock two years ago (basically Carter with many enhancements). My fuel mileage went up 2 mpg at normal driving and the drive-ability of my car (hi perf 455 Pontiac engine) was nicely enhanced. I still have a stock but rebuilt quad on the Chevelle (396) and would not hesitate to put either the quad or Holley back on the Poncho, but would want to research replicating the jetting in the Edelbrock as it works so well. Also, by now, most older Quads are in need of a lot of rebuilding, including worn throttle shafts, something most rebuilders do not bother to do, as well as checking the base to ensure it is completely flat. Also, if you are having trouble with this after a lot of adjusting and you happen to have the carb off check the intake manifold to make sure the carb surface on the manifold itself is flat as well.

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Old 08-05-2007, 04:30 AM   #8
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I used to have one of these too. There is a reason we called them rotten-chesters. I like my Holley a lot better. I did get my Rochester to stop leaking and did actually get it tuned right. The advice here is good. I had to double gasket the bottom to get the internal leaks stopped. ( not gas leaks, vaccum leaks !)
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Old 08-05-2007, 06:07 AM   #9
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Turn all 4 mounting nuts far enough to remove the carb completely, and replace it with a Holley!
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Old 08-05-2007, 06:58 AM   #10
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4 barrell quadrajet rochester

I was taught to overhaul those by a master, so I never had much trouble with them. Along with that goes the ability to identify wear problems that cause specific problems.
I liked the computer controlled quadrajets even more. Thye were a lot easier to set up properly.
However, carburators are a lost art. Worn throttle shafts and choke shafts(causes choke sticking) and those had to be addressed.
Our experience over the years was that it was hard to get a Holley to idle to our satisfaction, it rand like a scalded dog but just wouldn't idle.
Edlebrock makes a quadrajet knock off that idles good, runs good, gives almost as good a fuel milage as the old quadrajet (set up properly) and is very easy to overhaul by comparison (far fewer parts and adjustments).
I still have have a computer Rochester on my 84 Camaro and still get better than 28 MPG on the highway.
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Old 08-05-2007, 07:04 AM   #11
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the q'jets are good carbs. start with 1 to 1 1/2 open on needle adjustment, like previous post have said. drive the engine with a vacuum gage mounted near the steering wheel. as you go through the gears, up to speed, keep the engine vacuum above 7 1/2 inchese vacuum. at around 7 inchese the power valve open and admits 27% more fuel to the primary circuit. the extra set of venturi in the primaries meters the fuel more accurately. i driven 350 and 454 4x4 crewcabs with campers and got good fuel mileage by watching the vacuum gage at cruise speed and going up hill. good luck. ac
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Old 08-05-2007, 09:56 AM   #12
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I belong to the camp that considers the Q-jet to be a great carb. It is more complex than a Holly, and therefore harder to work on. Your average Bubba doesn’t want to learn how to set them up. Hollys are fine for racing, especially drag racing, but the missing complexity is devoted to drivability and economy enhancements. That said, I also consider the Edlebrocks to be excellent carburetors, the equal of the Quadrajet. I put one on my previous TV, a ¾ ton Dodge van and was very pleased with it.

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Old 08-06-2007, 09:34 PM   #13
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I rebuilt my QJet two years ago on the motorhome with a $30 napa rebuild kit and a great book from amazone on QJet overhauls) and it works fine now. safari57 is on the money with the idle mixture adjustment.

BTW - these screws are normally factory "plugged" so did someone before you drill them out?

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