# Quadra-Jet Conundrum



## BobG (Dec 20, 2006)

This is a question for anyone with significant Q-jet experience, but I think only Lars will have the answer.

Background: 
A few weeks ago, I decided to put the original Quadra-Jet back on the goat and ditch the holley. I got the Q-jet out and figured it needed a rebuild prior to installing, since it's been sitting in a box for about 14 yrs or so. So ... I went searching for info on Q-jets and found this neat site called "All the Quadra-Jet information I have found on the internet" aka All the QuadraJet Information I have found on the Internet. It has significant resources for info on setup and tuning of the Q-jet.

Conundrum:
There appears to be a conflict, however, in two articles dealing with the role and importance of the secondaries.

Article one, titled "How to Tune a Q-Jet" by Lars Grimsrud (Our Lars?) states that the primary jets provide most of the WOT performance, and that the secondaries act as a complement to the primaries only at higher RPM's. It further suggested that to affect WOT performance, the primary main jets are where you should start, as they have the greatest effect on WOT performance.

Article two, The Edelbrock 1910 tuning manual (the 1910 'performer' 850 cfm Q-jet owner's manual), states that the Q-Jet is a spread-bore, (I know that much), and that the secondaries deliver 75% of the air-fuel charge at WOT, and therefore to affect WOT performance, you should start with selecting the appropriate secondary metering rods.

Questions:
1. Who do I believe, the manufacturer, or a trusted technician?
2. Has anyone bought the Edelbrock race calibration kit? 
3. If so, can you tell me what primary jets / rods and secondary rods it contains? What springs?
4. I can get a replacement phenolic float. Is that any better than the original foam, or is it better and less prone to saturating? Does anyone make a nitrophyl or brass float?
5. Does anyone know of a source for already modified baseplates? My primary throttle bearing has a significant crack, and needs to have bushings installed. I can get it done by mail for 65 bucks plus postage, or I can buy the bushing set for 100+. 
6. Does anyone have any experience with buying an entire carb from one of these places like year one or summit racing? I've mentally added up what I'll possibly spend on all the pieces and parts and test equipment and such, and by the time all is said and done, not much has been said or done, but I would have been able to buy a rebuilt/calibrated Quadra-jet for what I'll have spent on the pieces, parts, and test equipment. The only thing I'd be out is the experience of tackling the beast and learing the best way to adjust it.

BTW, I would rather that this NOT devolve into a 'Holley vs Q-jet' discussion. My mind is made up, and I'm gonna try the Q-jet.


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## Old Indian (Jul 4, 2007)

Bob,

I've done some Q-Jet work. Both my 69 GTO and my 87 Cutlass have Q-Jets. The two best sources of written information have been Doug Roe's book and the Pontiac factory manual. Doug's book is very good but lacks some of the specifics found in the shop manual for a particular year, make and model. 

The comment about the effect the primaries have on WOT is RPM dependent. Until the engine RPM comes up, the primaries are doing the work. The secondary throttle opens with the primary throttle but until the air valves open, little to nothing is passing through the huge secondaries. There are three keys to the secondaries working properly. The rods determine fuel control, the rod hanger determines the rod position in the jet, and the the position of the hanger is controlled by the cam that is rotated by the air valve rotating to the open position. On the front passenger side of the carb is a choke pull-off. As manifold vacuum decreases, the pull-off allows the air valve to open. There is also a spring on the air valve shaft that affects the opening and closing rate of the air valves. The combination of the rods, hanger, pull-off and spring rate give you much more control over the rate of the secondaries opening and the fuel to air mix than a Holley ever will. That said, getting all of them to work together to give you a smooth application of power from half throttle through redline can be frustrating if each needs attention. The primaries compound the problem if the power valve is not operating smoothly or if any of the various fuel and air passages in the carb are gummed up, blocked or only partially blocked. 

Lars has provided some excellent recommendations in various posts on how to make the carb functional and then how to tune it for your driving habits. The best advice he ever gave me was to make sure everything else was working before pointing to the carb as the problem. If your ignition advance curve is out of whack, you can't expect the carb to overcome the problem. If you have vacuum leaks, you can't expect a device that works on slight variations in vacuum to work properly. 

I like both Holley and Rochester carbs. When I needed a replacement for my Olds, I got a rebuilt one from Summit. The unit was rebuilt by Holley. I had to do some serious swapping of parts from the old to the new carb, like secondary rods, hanger, pull-offs, idle load compensator, etc. but I was impressed that it idled perfectly right out of the box. That tells me that they bench flow tested it to set mixture and idle before shipping it to me. The throttle body to throttle shaft clearances were extremely tight. My primary reason for replacing the old carb was because the shafts needed to be rebushed. I would recommend the Holley rebuild. Summit is one of the few places that sell them and they are not available directly from Holley.

Hope this helps.
Bill


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## BobG (Dec 20, 2006)

Thanks, Bill, for the reply. At least now I know what I'd be getting if I bought a rebuilt. That's good AND bad, because one of the things I need for this one is a new idle compensator. mine's severely bent / warped and does not fully seat.

There's a carburetor shop across the river in Indiana that I found out about on a local board. They MAY be able to help me with some of the hard-to-find parts, and getting bushings installed.

I have read and followed Lars' comments from his "tuning for beer" post and have my distributor well set up.

I'm in the process of giving my Q-jet a thorough cleaning, which is how I came upon the fact that it needs some parts.

I have another question, though regarding the model info on the carb. I noticed, as I was examining my baseplate, that it had a number stamped on it that is similar in format to the model info on the carb. Is this just an assembly number, or should it match the number on the body of the carb? mine doesn't match, and if I take it to the number charts, it looks like it's from a 67 chevy, sort-of.

The number on my carb is: 7040264 VB
The number on the baseplate is: 7037174 and is cast in a circle just forward of the rear driver's side mounting hole. The part number is on the passenger side just to the left of the passenger side mounting hole and is 40794.

This info isn't critical to me, as I don't have a numbers matching goat, but I'm just curious if my base plate has already been replaced at least once.


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## Old Indian (Jul 4, 2007)

The base plate will likely carry the assembly number similar to the main body. The differences in the baseplates are minor but not insignificant. I have a table from the 69 Pontiac shop manual that shows what model carb was installed on which engine variations, but it does not list the baseplate numbers. Maybe the guys the carb shop will have additional references. The carb body number is going to define the primary jet sizes, the sizes of the air bleeds, etc. One significant difference in the primary circuits is the size of the power valve spring. These are matched to the engine during initial build. If you change your cam and it makes a significant change in your idle vacuum, this spring can make or break your idle, your transition from idle to power and ultimately the car's driveability. A carb shop can match the spring rate to the engine you will run it on. Rebuilders tend to build a single carb to fit more engines than Rochester did. That means compromises on some adjustments, sizes, and rates to fit a broader set of applications.

You asked earlier about float material. I've had good luck with most all aftermarket floats. The key here is to set them up correctly so they don't bind and the fuel level isn't so high that it sloshes on acceleration.

I was disappointed with my local GM (used to sell Olds) dealer who was unable to fix my carburetor related problems. All he could tell me was that my throttle shafts needed attention. He reset my timing while the car was there and he set it incorrectly. He was also not able to get me the carb I got from Holley through Summit. That is what drove me to understand them much better. I'm convinced that unless the guy who is servicing your carb has a bunch of gray hairs, he's lacking the experience to get it right the first time.

39 years of throttle action is a lot unless it was done a 1/4 mile at a time. The likelihood of the throttle plate being the original is slim unless the owners or maintainers never checked for vacuum leaks around the shafts. As the leaks get worse, it become harder to dial in the idle because there is too much air bypassing the idle circuit. You mentioned that you found a source for rebushing at $65. Can you share that one? I kept my old carb and I'd like to get it bushed. The locals look at me funny when I ask those kinds of questions.

Hope you get yours straightened out.
Bill


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## BobG (Dec 20, 2006)

Thanks again for the info. The Q-jet has been sitting in the Holley box since the Holley was installed, I think that was about 92 or so, so the Q-jet only saw 22 yrs of service, I'm guessing. It's never been overhauled, to my knowledge, as there is no evidence of epoxy on the infamous leaky tubes. I guess it's likely that the baseplate is original.

the place I found is called, appropriately, Tin Indian Performance. I don't know anything of their reputation, and haven't checked them out to see if I feel comfortable with them, but they're there, and at first blush, they seem to be legit.


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## Old Indian (Jul 4, 2007)

Thanks, Bob. Good luck getting your carb working the way you want.


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## VettenGoat (Jun 19, 2007)

Bob,

Haven't had much recent experience with Q jets of late, but back in the old days, one trick we used to do was to remove the metering rods (they come out together) and lightly file the tapered end. The extra flow made a noticeable performance difference in the RA III I was running (although I did later opt for a 780cfm Holley double-pumper). But in today's environment, I think you are wise to stick with the QJet; just give it a little tweak if you need to liven it up. Good luck//Mike


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