Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

90% of 'Carburetor Problems' are 'Ignition Problems'

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Buick73:
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.

Buick73:
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.

Buick73:
Pictures

Kenth:
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

Buick73:

--- Quote from: Kenth on January 06, 2026, 09:01:32 AM ---I wouldn't call Pertronix an upgrade, quite the opposite, as they have proven to be a source of endless problems.
--- End quote ---

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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