lars

As I've noted many times in my posts on this Forum, either buying a "rebuilt" Q-Jet carb, or having your Q-Jet "rebuilt" by a "mechanic", can be a real bad experience for many people. There are few people left who do a good job on these antique carbs, and most will get you hosed up pretty bad...

One of our Forum members recently sent his '69 Vette carb out to be "rebuilt" by a "professional" carb shop. When he got it back, it "had trouble idling" and "did not perform right." Let's take a look at what was done, and what needed to be done to correct the issues.

The first thing to be aware of is that the 1968-1969 stock GM intake manifolds were "hot slot" manifolds with an exhaust crossover groove under the carb to pre-heat the carb in cold climate conditions. The manifolds look like this with the "smiley-face" exhaust crossover groove running just forward of the primary throttle bores:


In order to seal this exhaust crossover and prevent exhaust gas ingestion into the intake manifold, a special grooved high-temp base gasket like this is required:


To prevent the exhaust gas from entering the bottom of the carb, a special stainless steel shield must be used on top of this gasket:


To sandwich it all together and make it seal up, a top 4-hole gasket is then used:


The entire gasket "pack" goes down like this to seal it all up:


Every professional carb builder is aware of this. Yet, the "mechanic" who built and delivered this carb provided our Forum member with the stainless shield and an open-plenum gasket. The open-plenum gasket cannot seal the shield against the exhaust groove: You can see the tracks of the massive exhaust gas leak directly into the intake manifold and over the bottom surface of the carb. This hot exhaust gas injection directly into the intake is much worse than a simple large vacuum leak, in that it can cause detonation:


Exhaust gas has been leaking right into the primary and secondary throttle bores:


The bottom of the carb shows the same evidence:


The carb itself had the tell-tale sign of a commercial rebuild job: The entire carb had been sandblasted and stripped of all coatings - down to bare zinc castings:


Some may think this is "pretty," but it's the first sure sign that things are screwed up:


First red flag was the carb's fuel inlet fitting. This fitting style is an over-sized, self-tapping fitting, used by non-professionals when the inlet threads are starting to go bad. A correct, stock fitting is shown below the carb:


The carb builder compounded the problem by not just jerry-rigging the self-tapping fitting into the carb, but he sealed it into the carb by coating it with JBWeld. Can you imagine being the first guy who intends to replace the fuel filter in this carb, pulls the fitting out, and ends up with a destroyed carb and a stranded car? This is what the inlet looked like:


One of the first things I pull off the carb during disassembly is the secondary rod hanger with the rods. The '69 Vette performance carbs used the 0.0400" medium-taper "BG" rods. These rods were unique to the Vette, and they are no longer available from any sources - they are difficult to find. Our Forum member knows that the carb was all original and correct when it went out for "rebuild" - the car has always been in the family, and he has had it since he was 16. But the rods I pulled out of the carb were not Corvette BG rods... the builder took the rare Vette rods and slapped in a cheap set of "AN" Caprice Station Wagon rods. Here you can see the huge visual difference between the correct "BG" rod (right) and what the "rebuilder" substituted ("AN"). This would have caused a massive lean condition and power loss at Wide Open Throttle:


With the top off the carb, let's take a look at the fine accel pump used in the rebuild.... This pump would have been completely inoperative:


One of the critical adjustments on a Q-Jet is correctly setting up the Power Piston Height (Adjustable Part Throttle, or "APT"). This is essential for proper fuel mixture at cruise and off-idle, and will affect whether or not the carb has an off-idle stumble. The power piston was set WAY low in the bore, producing a severe lean condition:


Compare that to where is should have been set:


To further cause a massive lean condition of the carb, the float level was actually set at .560" - yes, it was lowered to a position of over 1/2"..! It should be between 1/4" and .300" :


The float that was used was such poor quality that it was swollen, and would jam itself into the float bowl due to the swelling. Although everything was set up lean, this carb may actually have been flooding out. Notice float jammed between the venturi bore castings:


Some parts on a Q-Jet are just troublesome and irritating to install, so the builder just left the parts off. Here is the power piston and rod assembly. It's supposed to have a rod retaining spring installed:


Like this:


To make sure the needle seat did not leak, the builder glued it into the float bowl with JBWeld. You can see the JBWeld oozing out under the seat gasket:


Only way to remove the seat is to get the JBWeld so hot that it softens up:


...and then use Vice-Grips to get the seat to rotate just a little bit between multiple re-heats:


Seat removed:


Visual inspection of the Idle Fuel Restrictor Tubes shows evidence of tampering... a quick check with long inspection pins reveals that the carb builder has drilled out the tubes, making them grossly oversized. This will completely mess up fuel mixture during transition and at cruise:


And, again, there is no need to install all the pesky parts of the carb, so the secondary lockout lever was simply deleted:


Other items found during disassembly:

• The throttle linkage between the primary and secondary side of the carb had been bent & altered, jamming the linkage as the carb went into WOT
• Incorrect hardware installed in the airhorn - 4MV divorced choke carbs did not use lock washers on the airhorn screws. All screws sandblasted and stripped of their black phosphate coating

So the disassembled carb went in the hot tank, and when it came out, it became apparent that the carb had not only been sandblasted, but it had been painted with catalyzed urethane silver paint. Some of it came off in flakes, and other areas stuck really well. This looks like crap:


Even the steel parts were painted:


The entire carb had to be carefully re- bead blasted to get all the flaking paint off the surfaces - inside and out.

Once the carb was cleaned up, the damaged IFR tubes were extracted:


Side-by-side comparison shows how much they drilled out the tubes - a correct, stock .035" tube on the left, which I have opened up to .038" to allow the carb to run well on 10% ethanol pump gas. The altered tube on the right - it's about 100% richer than stock:


New tubes going in:


Back to a correct setup:


Correctly refinished carb assembled and ready for test. Compare to the "before" photos above:




Carb fired right up on the test engine. Here it's running cold and warming up:


Running hot with the choke wide open. How do you know it's running...? The choke pulloff is fully retracted, and the secondary airvalve rod is fully forward in the airvalve lever slot, holding the valves closed:


Air/fuel mixture right on the money at 14.6:1


Throttle response is great, and this carb is now running perfectly, as it should. There will now be another well-running '69 on the road again!



Lars

jim2527

Thanks Lars. I always learn something new in your carb rebuild threads.

OMF

It must be harder and harder to find a carb that hasn't been butchered (maybe that's too strong a term,) "freshened" so bad that you can still rebuild them properly.....that one seemed pretty close

Boomer880

See my 'junkyard' thread. I think Lars can fix any Quadrajet!
Sometimes ugly carbs might be the best for a rebuild. Mine was a PIA for Lars, but she sings like new now.

bradleyb66

I've got one of those 'pretty' carbs sitting on the shelf. I keep meaning to e-mail you some pictures to see if it's better off as a paper weight...it's a 79 carb, so not quite as rare as the 69's. The carb that you rebuilt for my 69 a couple of years ago is still running sweet!

lars

You're absolutely correct - the condition of the carbs I've been getting keeps getting worse and worse. "Scrap" carbs are now the new "rebuildable" carbs. Here's the carb I received from "Boomer880":

Shark Racer

Having about zero lathe experience, how hard would it be for someone who can work a mini lathe to convert some of these fatter tip secondary (and primary) rods to mimic the profile of a performance taper and tip?

lars

I've done that several times, but it's labor intensive. As certain models and years of these carbs become more rare and valuable, the cost to save them will go up to cover costs involved in actually fabricating parts for them. I am already fabricating power pistons and air bleeds from scratch - this is one of my custom made power pistons to replace the power pistons that the commercial builders destroy by cutting the stop pins off the bottom:



Lars

Shark Racer

That's amazing. I am envious of people who have the talent and the tools to do that.

Tampa Jerry

Lars: Great work as usual. On my 68 small block, I inserted freeze plugs into the manifold to block the exhaust gas from going under the carb. Since I live in Florida, I never need to worry about warming up the carb. Can't wait for your latest engine rebuild on the C-2 forum. Jerry

lars

You can install freeze plugs in the crossover groove, or you can tap it for pipe plugs to block the exhaust gasses from going under the carb. However, you still have to use the correct gaskets as shown in my initial post above due to the "bulge" in the manifold's carb mounting surface on the driver's side of the carb base: If you use a standard intake gasket, the groove with be exposed, and you end up with a significant vacuum leak. So the plugs still won't let you skimp on the correct gaskets...

Lars

Tampa Jerry

Lars: I am looking for the gasket set you referenced. I found a few listings for Buick, Caddy and Pontiac. Do you have a source or part numbers? Thanks. Jerry

CanadaGrant

On 68 and 69 big blocks GM used an aluminum intake made by Winters. Running hot exhaust gasses through cast aluminum was not one of GM's stellar ideas to say the least.... The intake below is off a 69 427/390 and the crack is across the exhaust crossover under the heat shield. It was not repairable by welding as the aluminum was like glass and would actually shatter from all the heat cycles. Also, if you try and use a later model Quadrajet carb with the stock base gasket and metal heat shield plate the vacuum leak will be so bad it won't run anywhere close to right. Different carb bases.

lars

E-mail me for a copy of my "Q-Jet Hot Slot Gasket" tech paper and I'll send you all the info with part numbers.

-or-

Check my post on the subject here:
https://www.corvetteforum.com/forums...d-problem.html

Lars