defiant1

Okay, I have read several posts that indicate tuning a Q-Jet to your specific application is recommended to achieve optimal performance.

But, where do you start? Are there links/resources out there that chart what type/size needle seats, primary/secondary metering rods, jets, etc. that are recommended based on the c.i. of the motor, transmission type, and rearend gearing, cylinder head mods, etc.?

I ordered Cliff Ruggles book on quadrajets to help make me a little smarter on the subject (still waiting for it to come in the mail).

At any rate, I have a Quadrajet that came off a 1980 Camaro (17080204 CPM Camaro, Canada, A/T 350,305 BTD 1980). It has been rebuilt. The previous owner had it installed on a 350 Chevy in a 1/2 ton p/u w/700R4 transmission. He complained that he did not get good gas mileage. He spent $$ on an Edelbrock in hopes that it would get better, suffice it to say he is still is not happy with his MPG and wants the Q-jet back. Too bad for him...

I understand that there is no "one size fits all" concept to tuning. But I am looking for a starting point so I can acquire the different rods, jets, needles necessary for install or at the very least compare what I currently have to what is recommended.

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Rickman48

First thing I'd do is check the primary throttle shafts for wear - any play is unacceptable! Since it was on at least 2 cars, it's probably worn-out.
The best way to correct this is to find a bushing kit, re-drill it and install the brass bushings, and ream to the correct size.
Most re-built/re-furbished carbs have had this done, and it's not for the unexpierenced!
Without this done first, you've a major vacuum leak, making everything else you do a waste of time!
Also, since that carb is partially a smog carb, all but WOT is going to be set too lean for a motor with any compression.
Spend some time and money on a professionally rebuit Q-jet for your motor - it'll be well worth it!

defiant1

The throttle shafts were done during the rebuild. So I am good to go there.

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Jamesbo

IMHO

You're not going to be happy with this carb. I am by no means an expert. But I believe soon some more learned people are gonna tell you exactly why.

joe_padavano

Just so we're all clear, any carb needs to be tuned to your specific application for best performance. The Ruggles or Roe books are definitely the right place to start. Of course, before starting any tuning, be sure the carb and the rest of the car are in the best possible condition (proper tune up on the engine, new filters, etc.). Ensure that in addition to the throttle shaft bushings, the float is set correctly, fuel pressure is correct, the choke is working properly, and the idle mixture is set correctly. I would also suggest that you start by installing the primary jets and metering rods that were stock for your combo. You should also either get familiar with reading spark plugs or get a wideband A/F meter.

Start with the primary side, trying to get optimum A/F under various part throttle conditions at different loads. Play with primary rods, jets, and power valve spring as appropriate. When you're happy there, move on to the secondary side, working with secondary rods and air valve wrap spring adjustment. Finally, play with accelerator pump adjustment and air valve dashpot bleed hole size as necessary.

greenslade

check this might help http://www.oldcarmanualproject.com/m...QJet/index.htm

defiant1

Joe, thanks for giving me an outline on how to proceed.

I appreciate the link.

BILL DEMMER

the first place i'd start is to get a quadrajet core from an oldsmobile that closely matches your application, why re-invent the wheel, olds did it for you. chebbys are notorious for having a big hole for an air bleed on the primary side. don't forget the power piston spring people, it makes a huge difference. finding primary rods for the late chebby carb. could be a drag too, most of the late qj's used very custom tapers on the primary metering rods. then there is the likelihood of that carb. having an auxiliary primary metering rod setup, and a few other things that will fight you most of the way. there is no need to make things harder than they have to be.


bill

defiant1

I emailed Cliff Ruggles and asked him nicely if the carb I was going to buy was a good idea (pros and cons) vs. the year specific (1970) version. He said go with the later year carb with the caveat being all the internals were serviceable and tuned correctly & I should have no problem.

So, I think I am fine with the core q-jet, right now I am trying to find other people's carb settings that are similar/matched to my car's setup. Looking to use other's past experiences and trials to determine my best course of action.

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bccan

What is your setup - engine, mods, trans, gears? That would help someone that may have or had a similar setup be able to put you in the ballpark for a baseline rod/jet combo.

defiant1

Specs:

'70 Cutlass S
Original SBO 350 (9:1 CR)
Stock #6 heads
Stock camshaft
Edelbrock Performer 350 Intake with 1" carb spacer
Pertronix Ignition
TH350 (rebuilt, solid shifting)
Rearend: 2.56:1 open (I will be installing either 3.23 or 3.42 anti-spin gear set however)
Tire/Wheel diameter: 28"
Dual exhaust
Stock exhaust manifolds

Other notables: New aluminum radiator, new water pump, new oil pump

The motor and drivetrain are solid and I really like how it performs currently (no oil leaks, good mpg, etc.). But, I think I am not getting the full performance out of the motor with the 2bbl setup and of course the 2.56 gears.

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bccan

My experience is mostly w/ "old style" Qjets >74 but you should be able to cross over to the "170xxxx" series cabs that use shorter rods. A good starting point for you would be the equivalent of a #70 jet, primary rods appx 48. You might want to come up w/ some rods each side of that - say, 45 & 50,maybe even 52. AY or similar secondary rods. Have setups similar to this in 3 different carbs on 3 engines similar to yours - 9-9.5 CR, 1 stk 72 cam, 2 slightly larger cams, 1 Performer, 2 stock intakes, all w' stk exh manifolds, 2.56 - 3.08 gears. Good mileage, good performance.

Now you should figure out what a corresponding calibration would be w/ the later style rods - may be the same designators, and that should put you very close first try. But like I said, I haven't done much w/ the later version carbs so triple check my info for relevance. You will have a lot of info & reference specs in those carb books to help you.

defiant1

Thanks for the information. This is the type of feedback I am looking for. I am sifting through online resources that list the different type of rods by year. And as you said, once I receive the carb book, it should be a little more clearer.

Thanks again

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olds 307 and 403

A late 70's 403 carb would be a better starting point. I believe my 78 403 carb had 44 rods and 73 primary Jets. My Holley re man Qjet has 71 jets and mystery primary rods. Cliff's book further explains the large single air bleed carb's. You need larger jetting and larger rods, if it is this type of carb. See what is inside and go from there. Jetting effects all throttle positions and rods part throttle. There is also the APT setting on top of that.

Warhead

I must agree with Bill.
Why re-invent the wheel.
A good, stock build for your application.
Good luck, on whatever you do.
Jim

MHENDERSON

I would get a core that matches your car and go from there!!