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Rebuilding a 1969 "Rebuildable Core" QuadraJet Carb
In my recent post about "Fake, Re-Stamped Q-Jets" here: https://www.corvetteforum.com/forums...jet-carbs.html
I showed some photos of a "rebuildable core" in addition to the 2 commercially rebuilt "fake" carbs. That thread was left with the conclusion that the "core" carb was actually the best carb for building a correct Q-Jet for the '69 427 "numbers-matching" engine. The "junkyard core carb" was verified to be a "real" 427 carb, in spite of its condition...
I am often contacted by people wanting to know what "kit" to buy in order to "rebuild" a carb that has been procured off eBay as a "rebuildable" carb. With these carbs getting older and rougher, the "rebuild" process most frequently involves more than just buying a "kit." Getting these original, old carbs back to running condition most often requires a stash of original parts, a little fabrication, some machine work, and multiple sources for new and reproduction parts. Let's look at the core carb that I talked about in my previous post:
This carb was sold as a "rebuildable core". It is a 7029215, which means it's a 1969 427 Corvette manual transmission carb. This is the condition of the "rebuildable core:"
The carb is badly corroded. The choke pulloff system is a 1971 system. The carb is missing the following parts:
Choke intermediate lever
Choke intermediate rod
Fast Idle Cam
Power piston rod hanger & spring
Primary rods
Secondary rods
Secondary rod hanger
Most hardware
Fuel inlet fitting
Inlet spring
Idle vent cover (aka, "doghouse")
Idle vent rod
Idle mixture screw & spring
Accelerator pump with spring and spring retainer
Secondary lockout lever
The inlet threads are stripped out of the casting, and the secondary rod hanger cam is broken. The primary jets are incorrect and damaged. But other than that, it's in great shape, and it deserves to be saved!
Carb torn down for parts inventory. With the part in a pile, I start throwing parts from my carburetor parts shed into the pile, making sure they are all correct, original 1969 parts. One of the parts that is getting hard to come by is the correct, original 1969 plastic fast idle cam:
The parts were all run through the hot tank, and then went through a de-rusting solution to get all the rust and corrosion off the components. Correct, original jets and rods were located, and the stripped inlet was machined and repaired with a stainless steel insert. With the detail parts repaired and brought back to their original condition, a detailed assembly process could begin.The choke plate and the secondary airvalves had to be removed to allow proper cleanup and to bring the parts back to their correct color and appearance:
Before:
After:
Before:
After:
Before:
After. The correct plastic "fat" fast idle cam can be seen in this photo. Most commercial builders throw these away and install a steel cam instead. I have seen the plastic cams in white, red, and black:
Notice that the airvalves and the choke plate are not "gold cad" like the commercial builders tend to build the carbs. The "pretty" carbs with "gold cad" airvalves and hardware are a sure tip-off that the carb is a hack-job. Look at what a commercial carb looks like - Rochester never a built a carb that looked like this. These 2 carbs are both complete junk - see my post link at the top of this thread for a description of how bad these commercial carbs are:
Actual Q-Jet hardware was black oxide coated, and choke & airvalves were silver cad with no yellow dichromate. Some later year Q-Jets had some yellow dichromate (aka "gold cad") dyed airvalves, but the yellow/gold color was very light - it did not look like Goldfinger had done the work... Q-Jet hardware was never cad plated - it was black:
Some parts, such as the idle vent wire lever, are not available from any sources, so you have to fabricate your own - I use stainless steel welding rod to create exact duplicates using an original as a check template:
With a little effort, and paying attention to correct colors and plating, even a rough "rebuildable core" Q-Jet can be detailed out to look like it did in 1969. These carbs were not flashy or beautifully gold plated when new. They were actually pretty boring... But it's amazing how good a correct boring carb looks when done the right way:
So keep these parts away from the commercial builders, and do careful and meticulous work when you work on your original parts. Even rough parts and components can be worth saving to preserve the history of these cars.
The 4296 is the 1969 L88 ZL1 850cfm Holley. There is nothing really different about it versus any other 4150 mechanical secondary Holley in terms of setup and tuning techniques.
Lars
The early Q-Jets use non-standard 7/8-20 threads for the inlets, and the later Q-Jets use non-standard 1"-20 threads. These sizes are not available from HeliCoil due to the non-standard sizes. There is a manufacturer who has tooled up to make these odd threads in STI sizes, but the tooling to machine, thread and install these inserts is also specially made tooling that they sell for very special prices... But the real trick is not forking out the money for the STI tooling. Rather, it's fabricating the holding fixtures and jigs needed to be able to machine and thread the STI threads into the carb exactly perpendicular to the existing gasket sealing surface. If you're off by any more than a fraction of a degree, the inlet fitting gasket will not seal, and you end up with an inlet fitting leak that destroys the carb. I have procured all this tooling, and I have fabricated the fixtures, to do this work correctly. The STI inserts, once installed, provide a permanent repair that allows use of the stock inlet fitting and the stock inlet filter system.
Here is the stripped thread area prior to repair:
After machining, STI tapping, and installation of the insert:
Lars, you mentioned that the 69 fast idle cam is hard to find. Is there an acceptable repro part? It seems like it would be easy enough to 3D print a functional part, in the correct color.
Unfortunately i was the one that was duped not twice but 3 times, with the shiny Frankenstein carbs. At the very lease i am grateful and amazed at the transformation of the one real core i was able to obtain. Thanks Lars for the amazing write up and great work. Hopefully this will prevent someone making the repetitive mistakes i have made.
What does "STI" stand for where you discuss the new threads and inserts? Or am I missing something obvious?
As Jim mentioned above, the "STI" designation is an industry standard for Screw Thread Inserts. There are specs for "STI" tap drill sizes, and the "STI" taps all meet a specific STI spec, as well as the inserts themselves. There are STI thread gauges to check the tapped threads, and there are MIL-SPEC documents for the correct installation of the inserts. If you look at any drill size tap chart, there will always be a separate column for "STI" tap drills. MIL-SPEC documents such as MS21209 define and control the STI parts and processes, and most inserts you buy at the auto parts store or through McMaster will actually meet the MIL-spec.
Originally Posted by Bikespace
Lars, you mentioned that the 69 fast idle cam is hard to find. Is there an acceptable repro part? It seems like it would be easy enough to 3D print a functional part, in the correct color.
There are no reproduction parts available to replace the '68 or '69 plastic fast idle cams. They are being replaced by steel cams, which are not the same. A friend of mine in the carb business, Steve at Carbs Unlimited, is actually working on a 3D printed part, and has a prototype made for evaluation. Material selection for the process is critical, since the cam sits right above the intake manifold exhaust crossover, and these parts are always thrown into a bucket of nasty-*** parts cleraner when the carb is rebuilt. The parts are also not just plastic: The originals were molded around a steel counterweight insert in the "fat" part of the cam, and the pivot point was a steel bushing that was molded into the cam. These parts would also have to be produced and pressed into the 3D part after printing. That eliminates the parts from being printed with the web-like internal structure - the parts have to be solid. So it's not as easy as it may seem to make a "real" part... I'll keep scavenging for originals...
This morning was test day for the rebuilt carb. I firmly believe that there is absolutely no way that anyone, including (...or especially...) the commercial rebuilders, can rebuild a carb and assure its functionality without actually testing the carb to verify its performance and operation. There are simply too many odd things that can go (or be) wrong with these antique carbs to assume that a meticulous rebuild will assure that the carb is right without testing it.
So Robert's carb was mounted up on the test engine. Cold-start and fast idle was good with the choke closed. Here is the carb actually running on the test stand about an hour ago:
But once the engine warmed up and the choke opened up, the idle air/fuel ratio was way too lean, even with the idle mixture screws backed out to their limit. I had noticed during initial inspection that the IFR tubes had a very small restrictor - much smaller than the big block carbs usually have. I had not opened them up during the rebuild process because the smaller tubes will sometimes work depending on how the carb is set up with the air bleeds. But even the small air bleeds in this carb would not compensate for the small IFRs, so the carb had to come off the engine for a little idle circuit rework.
Here the carb is torn down for a slight adjustment of the idle circuit.
In tearing it down I also found another little variation that needed to be corrected: The carb was originally missing 1 idle mixture screw. I happened to have a matched pair of original 1969 idle mixture screws, all cleaned up and ready to go. However, the mixture screws I installed were small block screws. Once installed in the carb, they are visually identical to the big block screws, but the screws are actually different. The big block screws have a much steeper taper, which allows more fuel flow at idle with fewer turns of the screw. The steeper taper also pulls more of the screw out of the idle fuel passage, allowing more fuel flow. A quick trip to the carb parts shed with the one original mixture screw allowed me to locate one 1969 big block idle mixture screw to create a matched pair of BB screws. Here you can see the difference between the BB (left) and the SB (right) screws:
All commercially rebuilt carbs I have seen always have the small block screws installed, which cause tuning problems on the big engines.
With the adjustments and parts replacements done, the carb was re-run on the test engine. Here it's running hot with the choke now wide open (you can tell it's actually running because the choke pulloff is fully retracted):
Idle speed is a stable 850 rpm:
...and air/fuel ratio is right on the number at 14.6:1 - a perfect setup with the mixture screws now set at 2-1/4 turns out. The screws have excellent adjustability range in both directions with plenty of margin:
Rebuild, setup and test is successful, so the carb is now on its way to Robert to be used where it should be on a '69 427 Vette!
There are no reproduction parts available to replace the '68 or '69 plastic fast idle cams. They are being replaced by steel cams, which are not the same. A friend of mine in the carb business, Steve at Carbs Unlimited, is actually working on a 3D printed part, and has a prototype made for evaluation. Material selection for the process is critical, since the cam sits right above the intake manifold exhaust crossover, and these parts are always thrown into a bucket of nasty-*** parts cleraner when the carb is rebuilt. The parts are also not just plastic: The originals were molded around a steel counterweight insert in the "fat" part of the cam, and the pivot point was a steel bushing that was molded into the cam. These parts would also have to be produced and pressed into the 3D part after printing. That eliminates the parts from being printed with the web-like internal structure - the parts have to be solid. So it's not as easy as it may seem to make a "real" part... I'll keep scavenging for originals...
Lars
Thanks for the update! We use Ultem 9085 for things that have to be strong (the same strength to weight ratio as aluminum). It's expensive, so thankfully ABS works for most applications.
I have no doubt that there was a recall on the plastic cams. Every one I have seen has been cracked, but I have never seen one actually come apart after the initial stress crack relieves the shrinkage tension. I'm sure most of these cracked cams were thrown in the trash and replaced with the steel cam, which makes them even more rare and hard to find... If the cam is cracked, but is still solid and not breaking apart after 50 years, it's probably not going to break under normal usage, so it's a cool item to install on an original carb.
This is what the original cams look like. You can see some of the cracking on the red cam:
This is a '69 replacement cam:
I like to keep these carbs "alive" with their original parts whenever possible - makes them look pretty cool when all the parts are "correct" and original.
Lars
Lars, It looks like you've got a lot of experience with Quadrajets. One of the problems with the early models was that the "freeze plugs" underneath the float bowl would eventually begin to leak. Back in the day when I was selling parts, there were two fixes available for this problem. One was a foam plug which fit between the throttle plate and carb body, the other, which was a much better fix, were aluminum plugs with o-rings that replaced the plug. I don't see this in your rebuild and am wondering if you haven't experienced issues with these or it's just not in your rebuild process.
The above narrative was not intended as an all-inclusive tutorial on rebuilding Q-Jets - it was just an overview of the current condition of many Q-Jet carbs, and a demonstration of how bad they can be and still be fixed. The majority of the actual rebuild steps were not covered in the above story.
The "freeze plugs" that were installed in the secondary wells are easily sealed with the foam seal plug - I install those in every carb I do. I also do a 12-hour leak-down check of every float bowl to verify that none of the other plugs leak: The primary well plugs will occasionally leak, as will the forward fuel inlet plug (this is more common than the secondary plugs leaking). These require machine work to correctly and permanently repair.
I say THANKS just for the education and information. I don't even have a carb on anything anymore, but I appreciate having this information available to read. It's amazing how well the manufacturers were able to make engines run with these fine mechanical and vacuum controlled contraptions. It is impressive how you are able to knowledgeably share information for us to absorb. Thanks.
03-05-2019, 09:39 PM
Rebuilding a 1969 "Rebuildable Core" QuadraJet Carb
