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Old 08-26-2002, 10:33 PM   #1
Ghostrider
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Default ZZ4 with Rochester QuadraJet carbs!

This may be an old topic, but my 1980 C-3 with ZZ-4 is mated with a 700R4 tranny and still has the old reliable Rochester QuadraJet (model: 17080204-149D-CPM) from the L48 set up. The carb performed badly at the last Dyno test where it ran too lean at about 4,900RMP (short of the rated 5,700 RPM . I only got 260 RWHP when the power curve dropped off with the start of pinging and lean running. I was told that this ol’ type of Rochester was even used on GM 427 or 454 CI engines but with different jets and fuel metering rods. I agree with carb guru: Lars Grimsrud (see Tech Tips) and want to keep my Rochester carb. A local expert also agreed and suggested trying a “couple of jets and fuelling rods”. My question:

Can I hit the nail right on, by immediately installing the correct GM jets and fuel metering rods?
If so what model numbers are they? Who has the correct set up?
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Old 08-26-2002, 11:31 PM   #2
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Default Re: ZZ4 with Rochester QuadraJet carbs! (Ghostrider)

You might try emailing Lars or Tom454 directly, either one of those guys know Q-jet tuning and can probably point you in the right direction.
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Old 08-27-2002, 03:35 AM   #3
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Default Re: ZZ4 with Rochester QuadraJet carbs! (Ghostrider)

I have a Lars rebuild on my ZZ4 and it runs great and very strong up to the 5800 redline. I have never had it dyno'd so I can't give you power numbers but when I get back from Japan (I am on a business trip and will be back in a couple of weeks) I'll check the stampings on my metering rods so you'll have a place to start....if you don't get the informations before I get back. I'd e-mail Lars and tell him you have a ZZ4 and he'll suggest what rods you'll need. I'd also maybe check your carb spacer between the carb and intake manifold. You may have a slight air leak that will cause your lean condition at high RPM's
John


[Modified by theandies, 1:36 AM 8/27/2002]
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Old 08-27-2002, 04:56 AM   #4
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Default Re: ZZ4 with Rochester QuadraJet carbs! (Ghostrider)

I wish that I had first hand information to share but I don't. I still haven't dialed in my Q-jet. I've pretty much parked it until I can find the time to dial in a few things that need attention on the car.

The answers that I received in the following two posts might provide you with some insight.

Q-Jet rod/jet combo. What are you using? http://forums.corvetteforum.com/zerothread?id=312539
Same question asked in the "Engine Mods" section. http://forums.corvetteforum.com/zerothread?id=312542

Have you read Lars' "How to Tune a Q-Jet" paper? Here's a link to it on Terry Rudy's CorvetteFAQ.com website. http://corvettefaq.com/c3/QJet_Tuning_Paper.doc
and Lars' "Jet Change Chart" http://corvettefaq.com/c3/Jet_Change_Chart.xls
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Old 08-27-2002, 10:27 AM   #5
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Default Re: ZZ4 with Rochester QuadraJet carbs! (Ghostrider)

Quote:
Can I hit the nail right on, by immediately installing the correct GM jets and fuel metering rods?
If so what model numbers are they? Who has the correct set up?
I don't think there is any *immediate* correct carb setup. If you get it, consider yourself lucky. I would, like mentioned above, email Lars and ask for a starting point. Then, buy some jets and rods just above and below what he recommends. Read his How To on Q-jets and start the trial and error testing.
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Old 08-27-2002, 10:48 AM   #6
clem zahrobsky
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Default Re: ZZ4 with Rochester QuadraJet carbs! (Ghostrider)

the carb comes with pri jets #71,pri rods # 42, sec rods CH .057 tips i would try CJ sec rods with .040 tips :chevy


[Modified by clem zahrobsky, 9:49 AM 8/27/2002]
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Old 08-27-2002, 09:58 PM   #7
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Default Re: ZZ4 with Rochester QuadraJet carbs! (Smokehouse69)

I did not want to bother him with stupid questions but I did. My problem is that I didn ot have A/F ratios. but here was his answer:

"Ghost -
Hi! Good hearing from you - thanks for the note.
The 1980 Q-Jet is jetted a bit on the lean side for emissions purposes, but can certainly be set up to satisfy your requirements. When you did the dyno run, did you get a printout of your A/F ratio so you know how lean it really was? If you have the numbers, we can see how many percent you're off and re-jet the carb by this percentage. This will get you really close to the ball park on the first re-jet attempt. If you don't have any numbers, I can recommend a combo that will improve your setup, but you'll be in a little bit of a trial-and-error mode. Tag me back with any info...
Lars"
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Old 08-27-2002, 10:00 PM   #8
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Default Re: ZZ4 with Rochester QuadraJet carbs! (theandies)

I can't wait that long but I will post the results. Thanks anyway.
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Old 08-27-2002, 10:03 PM   #9
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Default Re: ZZ4 with Rochester QuadraJet carbs! (clem zahrobsky)

This is good info, that I will try, as long as it is for a ZZ4 or equivalent setup.
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Old 08-27-2002, 11:04 PM   #10
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Default Re: ZZ4 with Rochester QuadraJet carbs! (Ghostrider)

these secondary rods are used in carbs with a open face air cleaner and may even be rich but you are better off to start off rich than lean. if too rich try DA which are .044 dia. :chevy
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Old 08-29-2002, 12:06 AM   #11
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Default Re: ZZ4 with Rochester QuadraJet carbs! (clem zahrobsky)

Thank you all. FYI, here is Lars' final suggestion:
Ghost -
Since you don't have any A/F ratios to play with, we're shooting just a bit in the dark, so any changes made need to be a little bit on the conservative side to keep from "overshooting" the mixture.
The stock jetting on your 17080204 carb is:
71 main jet/42 Primary rod/"CH" sec rod (.057")
This produces cruise and WOT metering areas of:
2.57/3.43 respectively.

As a first step, I would change the primary metering to a 73/44 combination. This will bump your metering areas up to:
2.66/3.65 combination.
This richens you up 3.5% at cruise and 6.4% at WOT. If this makes the engine respond better, and you're headed in the right direction, go to a 75/46 combo. This will put you at: 2.75/3.88
This is a 7% & 13% increase respectively, and should put you solid into the 13:1 range at WOT.

As a final tweak, pull out the "CH" secondary rods and drop in a set of "DA" rods. The DA's are .044" diameter compared to your .057 CH sticks.
This should give you a bit to play with and get you in the ballpark with throttle response and power.

Lars
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Old 08-29-2002, 12:06 AM
 
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