Ghostrider

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?

Smokehouse69

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.

theandies

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]

MIKER

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

BeaterShark

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.

clem zahrobsky

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]

Ghostrider

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"

Ghostrider

I can't wait that long but I will post the results. Thanks anyway.

Ghostrider

This is good info, that I will try, as long as it is for a ZZ4 or equivalent setup.

clem zahrobsky

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

Ghostrider

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