General Category > Quadrajet Carb Talk and Tips
What are these holes for?
77cruiser:
Where's Tuner?
Cadman-iac:
Still curious about how the size of the main nozzle air bleeds affect the AFR, and the relationship between these and the main jet size.
Cliff?
Cadman-iac:
I couldn't sleep tonight thinking about this carburetor, so I was posing some questions online for AI to answer and here's what I've learned.
In a Rochester Quadrajet carburetor,
the main nozzle air bleed is effectively the final stage of air bleeding, where the air/fuel emulsion is discharged into the venturi, while the upper and lower main circuit air bleeds (also known as emulsion bleeds) are earlier stages that control the initial mixing of air and fuel within the main well.
Difference in Function
Upper and Lower Main Circuit Air Bleeds (Emulsion Bleeds): These bleeds are located within the main well of the carburetor body and air horn. Their primary role is to introduce a calibrated amount of air into the fuel column as it rises from the main jet, creating an emulsion (a mix of air and fuel). This emulsification helps to decrease the fuel's density, promotes better vaporization, and helps "lift" the fuel into the airstream. The size and number of these bleeds shape the fuel curve in the mid-to-high RPM range by varying the air mixing as the fuel level in the well changes with engine speed. The upper bleeds have the most impact, while the lower ones have less effect.Main Nozzle Air Bleed (Discharge Nozzle): The final discharge nozzle (sometimes referred to as the "high-speed bleed" in a general sense, though it's the exit point) is where the air/fuel mixture exits into the boost venturi. While the entire passage acts as an air-bleed type system, the primary function at this point is to discharge the already-emulsified mixture effectively into the high-velocity air stream in the venturi. The final bleed size here fine- tunes the very top-end, full-throttle air/fuel ratio.
Effect of a Larger Main Nozzle Air Bleed
A larger main nozzle air bleed (or increasing the size of the final high-speed bleed restriction if applicable) introduces more air into the system or modifies the discharge pressure characteristics. The general effect is a leaner air/fuel mixture, especially at higher engine speeds and loads.
Leans the mixture: More air mixing with the fuel reduces the overall fuel density being discharged, leaning out the mixture.Reshapes the fuel curve: Air bleeds affect the shape of the fuel curve across the operating range, rather than just shifting the entire curve up or down like a main jet change. A larger main nozzle bleed will lean the mixture primarily in the upper RPM and high-load ranges.Delays system activation (to a degree): Larger bleeds can slightly weaken the vacuum signal in the fuel well, potentially delaying the point at which the main system fully activates or becoming more sensitive to engine load.
Pull-Over Enrichment System and Air Bleed Size
The Quadrajet's main metering system uses tapered metering rods that move in and out of the main jets based on engine vacuum (controlled by a power piston). The rods are pulled down into the jets by high vacuum (lean cruise mixture) and rise out of the jets under low vacuum/heavy load (rich power mixture).
More to follow, too much for one post.
Cadman-iac:
--- Quote from: novadude on October 15, 2025, 05:13:01 AM ---Bumping this to the top, as I am also curious about how these bleeds affect fuel curve.
--- End quote ---
--- Quote from: 77cruiser on October 21, 2025, 06:03:35 PM ---Where's Tuner?
--- End quote ---
Continuation from above:
The "pull-over enrichment system" (also known as the power system) works with this mechanical/vacuum system, but it does not change the main nozzle air bleed size itself. The air bleed sizes are fixed (or manually tunable during a rebuild). The enrichment is achieved by:
Varying Jet/Rod relationship: The metering rods (which have different diameter steps) move up and down in the fixed-size main jets to change the fuel orifice area.Air Bleed Interaction: The pre-calibrated air bleeds then interact with the fuel flowing through this variable-area jet/rod system to maintain the desired emulsion and fuel curve at different loads and RPMs.
In short, the pull-over enrichment system uses the existing, fixed air bleed design in conjunction with variable fuel metering to achieve the required enrichment, rather than changing the size of the air bleeds themselves.
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Now, here's how I think this ties in with what I've been seeing on the AFR gage.
My carburetor is originally from a Cadillac, and it has the POE system, or rather it used to.
In an effort to make it like a Chevrolet carburetor by matching the specs, I did remove the tubes for the POE, thinking that it was affecting the results. Eliminate the POE, eliminate the variables, right? Uh, not so sure now.
The problem that I see now is that it's running extremely lean throughout the rpm range, so apparently the POE system was kicking in and supplying the extra fuel.
These larger main nozzle air bleeds are definitely affecting the AFR.
As I see it, I either need to reinstall the POE tubes and recalibrate with them in place, or start over with huge jets and try to get things under control with calibrations to the idle circuit and the main bleeds, since I see no way to change the size of the nozzle air bleeds. Unless you guys know of a method to do so?
I think I'll be practicing on some old junk Q-Jet cores to figure out if you can change the nozzle bleed size.
What a pain in the backside!!
I wonder if they make those set screws in a 4-40 or a 4-48 thread?
77cruiser:
Did you try raise the float like Novadude mentioned.
Here the tiny setscrews.
https://www.mcmaster.com/products/screws/set-screws-2~/thread-size~4-40/brass-cup-point-set-screws/thread-size~2-56/
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