Quadrajet Differences

[truckster]

The engine I dropped in the Blazer when I got it three years ago already had an Edlebrock 1405 on it. I've been wanting to go back to a Quadrajet for about two weeks less than the three years.

I've got two Quadrajets sitting around that I could rebuild. I haven't run the numbers to see what they were orginally on. Both look pretty good around the throttle shafts. BUT- I'm getting lazier in my old age, and I'm thinking buying a reman might be less time-consuming and cheaper than rebuilding either of these.

I found a site that lists a Buick Quadrajet 75-76 for 350 to 455 CID for $139. So the question I have is: What are the differences between the Buick and Chevy carb, and will a Buick carb work on my stock 350 SBC?

Thanks.

[Crabass]

To expect any carb to work equally well on a 350 and a 455 is a bit of a stretch, ya think?

Unless done by one of a very select few re-builders, the "remanufactured" sticker should be considered the kiss of death. Your safest bet is to find a good, unmolested carb, do some research, and have it done by one of the Q-gurus. QJets are good when they're right, but there is a lot than can be "not right", which is why they got the bad rep.

Make sure the fuel inlet is where you want it, there are differences.

A truly lazy person would make a little effort to get a good carb, and avoid all the trouble that would have to be dealt with for a bad one. Did not mean to sound preachy, but as one that is as lazy as any I can tell you from experience that my recommendation is by far the easiest way to go.

Here's an excerpt you might find interesting, from another forum:

More Q-Jet Problems with Commercial Carbs Described
Forum member ------------ recently sent me a carb he procured via the internet (correct me if I'm wrong, Bob, but I think it was an eBay purchase or something similar). I want to use Bob's carb as an example of all the things that can be wrong with a carb, and why you simply can't just disassemble a Q-Jet, slap a kit in it, put it back together, and expect it to run right.

In all my papers and postings, I always warn people against buying "commercially rebuilt carbs," and I keep getting the question from people, "What's wrong with a commercially rebuilt carb?"

Bob's carb was a commercially rebuilt carb that had been recently rebuilt by a "commercial carb builder." Whoever bought it put it right back up for sale...

Here are the issues I found with this "near-new" 1975 Q-Jet rebuild and some of the things that had to be done to fix it:

Bent APT needle. Also incorrectly called the "altitude compensator," this device controls part-throttle mixture using an adjustable needle in a 3rd "jet" in the float bowl. The needle had been bent during rebuilt, causing the needle to not be engaged in the jet. This resulted in a "full rich" condition that somebody tried to compensate for with other "tuning" methods. Read on...
Lead plugs installed in idle air bleed holes. This is a common practice by most commercial builders. The Q-Jet uses an idle air bleed system that allows idle air to be pulled into the intake just below the throttle blades. By allowing a controlled amount of air to bypass the throttle blades, the throttle blade opening can be decreased, allowing effective use of the transition slots in the throttle plate. Plugging the holes for the idle air bleed results in the idle speed screw needing to be cranked up to the point that the transition slots are excessively exposed, causing off-idle stumble and hesitation.
Extreme rich jetted - damaged #78 jets. To compensate for other setup issues, the builder had installed #78 jets in a carb that should have #72 jets. In addition, they had slipped a bit with the screwdriver during assembly, damaging the jets and the fuel flow.
Throttle plate loose. The screws attaching the throttle plate to the fuel bowl were loose due to the factory-installed lock washers being deleted. This caused vacuum leaks at the throttle plate and a lean condition.
Severely bent primary throttle shaft & lever. The primary throttle shaft was bent, causing binding and sticking of the throttle.
Stripped inlet threads. Common problem on a Q-Jet. But in this case, the builder had installed a self-threading oversized inlet fitting, deleted the fuel filter and filter spring, and poluted the carb with metal debris from the self-tapping fitting.
Bent & destroyed secondary lockout lever. In an attempt to "adjust" the secondary lockout lever, the builder had bent the lever to the point of breaking the tang off the lower portion of the lever. The secondary engagement tang was broken, and the rest of the lever was severely bent and unusable.
Sandblasted finish. Common prcatice with the comercial builders: The entire carb had been sandblasted, removing and destroying all finishes and corrosion protection on aluminum and steel surfaces. Result is that aluminum surfaces are covered in white oxidation, and all steel parts are badly rusted. The carb had to be acid dipped and chemical conversion coated to bring back some of its correct finish.
Secondary airvalve loose & sticking. The builder had ground the staked feature of the retaining screws off, resulting in the screws loosening and jamming the airvalve.
Incorrect float installed. Brass floats don't cut it on a Q-Jet. Always use a NitroFill float.
Extreme low float level (.620"). To compensate for other issues, the float level had been dropped to an extreme .620" level. Correct level is .420. This low float level assured that the carb would not operate on the idle circuit, and the carb was idling on the main discharge circuit once the idle speed screw was turned in far enough.
Incorrect power piston spring. Common commercial rebuild problem: They install a "generic" power piston spring. This may work okay on a truck, but it sure doesn't work on a Vette. GM just discontinued the "Corvette" power piston springs 2 weeks ago...
Stripped threads in bowl for throttle body attach screws. Another common problem with these carbs is that they tighten the screws so far that the threads strip out. You have to have a handful of 10-32 HeliCoils any time you try to rebuild a commercial carb.
Bent primary rods. The tips of the primary rods were bent from incorrect attempt at installation into the jets. This eliminated any fuel metering control.
Accel pump rod broken. The commercial builders use junk parts from junk carbs. The rod attached to the accel pump lever was broken off at the top "jog," and the builder just installed a snap retainer rather than replacing the rod.
Bent primary throttle blades. In an attempt to get the throttle geometry right, the builder actually bent the primary throttle blades at an angle. This resulted in the primary throttles not fully closing. With the way the rest of the carb was set up, this was probably irrelevant, since the carb needed all the throttle opening it could get to idle at all..
Secondary throttles set to open over-center. Another common "speed trick." The secondary throttle on a Q-Jet should not open past vertical. In fact, they need to stop just short of vertical in order to produce most effective airflow through the secondary side.
Broken throttle plate. A common issue on the commercial carbs is that the builders install throttle shaft bushings in these carbs, whether they're needed or not. Very few Q-Jets really need throttle shaft bushings, and most people who install them screw up the install. In this case, the builder bored the throttle plate for oversized bushings and broke the throttle plate in the process. No problem - they just smacked the bushings into the broken throttle plate, left the massive vacuum leak unresolved, and let 'er go. The plate had to be repaired with correctly installed K-Liner type bushings.
Plugged idle fuel tubes. The smallest metering orifaces in a Q-Jet are the Idle Fuel Transfer Tubes. These tubes are pressed into the carb, and only the top oriface of the tube is visible in the top of the float bowl adjacent to the venturies. Commercial builders often pull these tubes out and install "generic" tubes with incorrect orifaces in the top and bottom of the tubes to compensate for the other "tricks" they do to the carb. In this case, the builder had installed tubes with lower orifaces too small, and these had been plugged up with the metal debris from the oversized inlet fitting. There was no idle fuel flow possible.

These commercial carbs obviously create some unique tuning and setup problems, and they are contributing to the notion that "Q-Jets are junk," since they never run right. If you have one of these carb, be sure you look critically at every aspect of the carb during any setup and rebuild process - you cannnot simply "install a carb kit" in one of these carbs and have it run right...

Have fun tuning!

[Wrenchbender Ret]

The main difference between the BOP carbs & the Chev is the BOP fuel inlet is straight forward & the Chev. & cad. come in from the side. The best one to work with is the late 70's early 80's with the round choke. If it does not have an electric choke you can put the electric choke coil in it. Then it will work on any manifold. Like Bass says look for an unmolested one (And don't molest it). They are easy to put a kit in & the satisfaction of doing it yourself is rewarding. Again like Bass says the over the counter remans are seldom right.