Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: Buick73 on January 06, 2026, 08:00:26 AM
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PART ONE of TWO (Character Limit)
My car (which has been in the family since brand new) is a 1973 Buick with 455-V8, 8.5 compression, Quadrajet carb (7042240), THM-400, 2.73 rear gears. Everything is as close to stock as possible, except no EGR (vac line plugged with tight-fitting ball bearing) & vacuum advance directly to ported source on carb. Trans rebuilt in 2013? Shift kit installed. Engine rebuilt in 2018 (30 overbore- would have made 20, but pistons not available at the time), stock cam, & points distributor upgraded to PerTronix. Approx 9,500 miles since rebuild. Carb rebuilt in Sept 2024.
Transmission seems completely normal. While I'm cruising (say at 45, 50, 60 mph), the car is normal under all conditions. I could press the accelerator lightly to increase speed slightly; if I approach an incline & give it a little throttle, it climbs easily; or if I floor it, yes transmission will downshift, secondaries open seamlessly, & car goes into passing gear- if I let up on the throttle, transmission upshifts & normal driving resumes.
Mechanical advance starts just above 1,000 rpm & maxes at about 2,800 or 2,900 rpm with 26 degrees. Initial 8 BTDC, so 34 total. Always run 93 octane [(R+M)/2] fuel, no ethanol (when possible).
In park, steady 19 inches of vacuum, idle 950 rpm
In drive, steady 15 inches of vacuum, idle 650 rpm
Up until about a year-and-a-half ago, car was completely normal. If I pushed accelerator too far (from a stop), the wheels would spin.
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PART TWO of TWO (Character Limit)
Now, I'm chasing an ongoing off idle/ low rpm hesitation- a flat spot/ sluggish from idle to about 1,500 rpm. Once past about 1,500 rpm, car is fine/ normal.
This presents in one of two opposing ways depending on scenario.
1. Off idle (at a dead stop): I am limited to a part-throttle acceleration, which behaves completely normal. I cannot achieve a WOT from a stand still. If I floor it from a stop, car moves forward as if it's in 3rd gear.
2. Low RPM (after slowing down enough to make a tight turn- now moving about 5 to 10 mph): it's the reverse of the above scenario. Part throttle attempt at acceleration is sluggish/ flat (again feels like it's in 3rd gear). Car dogs (particularly bad if I am now climbing a hill). However, if I turn the corner, and floor it (WOT), secondaries will open, & car will accelerate full power up the hill.
Both scenarios present while car is <1,500 RPM (the common thread to both scenarios)- though they present in contrasting ways depending if I'm moving/ coasting very slowly (point #2 above), or not at all (point #1 above). It's driving me nuts.
Accelerator pump on carb is verified & fully functional (& connected to inner hole- bigger shot). Secondary windup spring is 7/16 turn past contact, per spec. Primary jets/ rods: 73/44. Secondary hanger ‘K’ with ‘CT’ Rods. Screwdriver in vent tube test passes. It moves easily downward and springs upward (smoothly) with engine off. With engine running, a light screwdriver springs upward with throttle application.
Engine runs well during warmup; choke steps down in less than 5 minutes (dependent on ambient temperature). Then with first throttle blip, it comes fully off (90-degrees).
Current vacuum advance can is a ‘B9’ (7-9 in. tip in with 10 degrees advance @16-18 in). Verified to work as stated and hold vacuum with handheld gauge.
Any advice would be appreciated.
Thank you.
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Pictures
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I wouldn't call Pertronix an upgrade, quite the opposite, as they have proven to be a source of endless problems.
One of the biggest faults with them is the mechanic/enthusiast not following the instructions when it comes to the electrical wiring.
I've lost count of everyone I've helped with this, as they used the electricity for the ignition coil (which was intended not to burn the points tips), this electricity has reduced current and voltage, which gives a reduced spark on the spark plugs if used for Pertronix (or HEI).
This means that the engine will NEVER work as intended.
Make sure to connect the correct voltage to the ignition coil and the Pertronix will at least have a chance to work as intended (until it breaks).
For my part, I would never replace a well-functioning mechanical ignition system that has worked well on hundreds of millions of cars with some "modern" electronic gizmo that is not up to par.
FWIW
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I wouldn't call Pertronix an upgrade, quite the opposite, as they have proven to be a source of endless problems.
In looking at my pictures up close, does that look like a magnet on the distributor plate? I hadn't noticed that in person.
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In looking at my pictures up close, does that look like a magnet on the distributor plate? I hadn't noticed that in person.
Yes it does look like a magnet fell out. I've had a pertronix installed on my Vette since 1998. It's performed flawless all these years. I did wire it and the coil to full 12V by way of a relay. I didn't want to cut up the harness. The original ignition wire now triggers the relay. We all know stuff made nowadays is basically junk. I've read where their products are not what they once were, I doubt I would get that kind of life out of a new one. The same with points, A set that will rev past 5K is upwards of $80. I recently rebuilt a spare distributor for the Vette (tach drive) with points and installed to test it out. It works great but 5200 RPMs is all it will do before I get point bounce. And it may just be my imagination, but I swear it pops off quicker with the pertronix set-up.
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…looks like the wrong carb…every 7042240 I’ve built does not have that early style air lid…1972 455 also have a slash cut vent tube…pix#4 seems to show the secondary air valve linkage might be incorrectly adjusted….also check the front dashpot and verify it is functional.
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I wouldn't call Pertronix an upgrade, quite the opposite, as they have proven to be a source of endless problems.
I am with you 100% on this. I had tried Pertronix ignitors on 2 different cars, and I ended up going back to blue streak points on both. One of those left me stranded and required a tow. IMO, they are junk.
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There is more than one Pertronix Ignitor version for a Chevy points type dist offered. I have the original #1181 version which has a magnetic wheel trigger rather than one of the lobe sensing units. I would trust the #1181 much more than the others. I've had #1181's in two different Chevelles over the past 26 years with no problems. I've also installed them for others with no problems I've heard of.
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Seems like some people love them, and others have issues. The original 1181 is the one that cause me problems.
For my Nova, I have a somewhat unconventional points distributor conversion. I am using an old hays stinger points plate, reluctor wheel that presses on the points cam, and 70s mopar pick up in a '63 327 distributor. The pick-up triggers a remote-mounted 4-PIN HEI module that is mounted on a S10 heat sink and hidden out of sight.
This give me stock points distributor appearance but eliminates points using OEM reliable electronics. This is not a practical solution for everyone though, as Hays hasn't made these parts in decades.
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…looks like the wrong carb…every 7042240 I’ve built does not have that early style air lid…1972 455 also have a slash cut vent tube…pix#4 seems to show the secondary air valve linkage might be incorrectly adjusted….also check the front dashpot and verify it is functional.
The carb is indeed stamped with the '7042240'. As I recall, the top has been replaced at some point in the distant past. Yes, I know the secondary linkage is weirdly bent. (The problem was that it was dragging the choke mechanism). It's not perfectly angled, but it does work as intended- pulls the secondaries shut at high vacuum, dashpot fully releases in about 2 seconds, & secondary air valves open.
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So, I understand that people are divided on rather the Pertronix is a 'good thing' or a 'bad thing'. But my original question was about the hesitation issue (please see original posts).
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The carb is indeed stamped with the '7042240'. As I recall, the top has been replaced at some point in the distant past. Yes, I know the secondary linkage is weirdly bent. (The problem was that it was dragging the choke mechanism). It's not perfectly angled, but it does work as intended- pulls the secondaries shut at high vacuum, dashpot fully releases in about 2 seconds, & secondary air valves open.
…check the replaced air lid and verify it wasn’t replaced with the Buick 750cfm version, as those have the upper idle air bleeds in the air lid….the 800cfm version(7042240) do not have them in the air lid (unless custom modified)….Good luck.
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…check the replaced air lid and verify it wasn’t replaced with the Buick 750cfm version, as those have the upper idle air bleeds in the air lid….the 800cfm version(7042240) do not have them in the air lid (unless custom modified)….Good luck.
What would that do?
Please see picture.
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….open the choke door and look into the primary side….if you see only the two main air bleeds it is “factory”correct, if there are four( 2 main plus 2 idle air bleeds it is not “factory” correct….post a picture of the primary side with the choke open.
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So, I understand that people are divided on rather the Pertronix is a 'good thing' or a 'bad thing'. But my original question was about the hesitation issue (please see original posts).
An engine must be in good mechanical condition (cylinder pressure) and have a well-functioning ignition system for the carburetor to do its job as intended.
A point breakerless ignition system where the ignition coil has too low an operating voltage will produce too weak a spark at the spark plugs and the engine will hesitate when the throttle is applied.
So, do you have good cylinder pressure and korrekt voltage to the ignition coil for the ignition system used?
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…check the replaced air lid and verify it wasn’t replaced with the Buick 750cfm version, as those have the upper idle air bleeds in the air lid….the 800cfm version(7042240) do not have them in the air lid (unless custom modified)….Good luck.
Only 1967 Buick Quadrajets has the upper idle air bleeds in the air lid.
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An engine must be in good mechanical condition (cylinder pressure) and have a well-functioning ignition system for the carburetor to do its job as intended.
A point breakerless ignition system where the ignition coil has too low an operating voltage will produce too weak a spark at the spark plugs and the engine will hesitate when the throttle is applied.
So, do you have good cylinder pressure and korrekt voltage to the ignition coil for the ignition system used?
Engine was completely rebuilt in 2018. 30-overbore (would have made 20, but pistons not available at that time). Stock cam. All cyllinders with about 180 psi. Coil voltage at 12V to distributor (no resistor).
Yesterday, I replaced all the ignition components- new 0.040-gap plugs (see picture of old ones), wires, distributor cap, rotor, & a new magnet wheel (one had fallen out).
While into the distributor, I noticed springs were worn/ stretched; weights were floating outward & not returning to fully retarded positions. I have a pack of various springs for it.
I put the stiffest ones in just to set initial timing (10 degrees BTDC). Then, I started experimenting with softer ones (mix & match). When I found the ones that I like for driving, I checked my results with timing light. I have 26 mechanical (so 36 total), all in at 3,000 RPM.
I also swapped out my vac advance (B9) for a B27 (5-7 inHg tip-in), 18 total. This is closer to the original spec sheet (please see attachment) than the recommended 'B1' (8-11 inHg tip-in), 16 total.
Carb rebalanced. I found best vacuum at 2 3/4 turns from seated. I returned both to 2 1/2 turns for best lean idle.
OVERALL RESULTS:
1. Part-throttle hill climbing has resolved. I can climb a hill easily without having to floor it.
2. Off-idle hesitation has improved, but is still there. Before, I was limited to about 25% throttle from a standstill. Now, I can push about 50%. Car accelerates smoothly & quickly 'gears-up'. I can feel mechanical coming in. Vacuum is probably contributing too after a few seconds as vacuum level rebounds with speed.
CURRENT QUESTIONS:
1. How do I improve the off-idle response? If I floor it from a standstill, car dogs forward for about 2-3 seconds, then winds up & starts moving (kind of like if I had just started with 50% throttle, except now I'm 2 to 3 seconds behind). Please see 'Results #2' above.
2. I've considered:
(a): Swapping more initial in exchange for limiting mechanical to maintain overall total.
(b): Installing softer distributor springs to bring advance in even quicker.
I currently don't have any knocking under any driving conditions, but I know I would have to be close with 36 total & another 18 vacuum. However, I never cruise at 3,000 RPM (that would be more than 90 mph for me). So, I would never see the technical 54 degrees.
Any other carburetor tuning I'm missing?
Thanks again.
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To start with i would fix the mashed slot on air valve shaft and remove the secondary shaft lock out lever.
Then i would use #75-#76 main jets and CV, AU or CK secondary rods for better throttle respons.
HTH