General Category > Quadrajet Carb Talk and Tips

Which of these two quadrajets is the best candidate for a rebuild core?

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cal30sniper:
I’m looking for some feedback on which of the following carb cores I have on hand will make the best rebuild candidate. Both carbs are complete and fully functional, but both have been sitting for quite some time and both are in need of a full rebuild.  I plan to use Cliff’s HP kit on whichever carb I select.

First, the application:
-3200lb 1977 Plymouth, manual brakes, manual steering, no A/C
-318 factory roller cam motor, swirlport 302 heads, 9.2:1 compression ratio
-Lunati 20200714 hydraulic roller grind (207/213 duration at .050, 258/264 advertised, .485/.485”” lift, 112LSA)
-Magnum exhaust manifolds (think small headers) with free-flowing 2.5” dual exhaust
-Weiand 4bbl spreadbore intake (Action Plus) with 1” phenolic carb spacer and heat shield
-Early magnum air cleaner with ducting to fenderwell inlet
-46RH overdrive auto with lockup converter and 3.55:1 rear gears

I estimate the combo will be around 275 crank hp with near factory idle characteristics (should be 14+Hg at idle with a slight lope). Application is a commuter that will see 70-80 miles of mixed highway and city driving daily. Altitude here is 5500’, and it will rarely if ever see altitude below that. It may see up to 8-10k’ on occasion, but not on a regular basis. The car regularly sees morning start temps of 20F or below during winter operation. I will be maintaining the hot air crossover under the intake due to these cold operating conditions. The Magnum manifolds I’m using don’t have provisions for air cleaner heat, but I may add a hookup to use the hot air tube and vacuum controls for air cleaner heat during winter operation. It also sees high 90s and occasionally low hundreds during summer operation (high desert environment – hence the phenolic spacer and heat shield).

The carbs in question (please excuse my rough measurements, still waiting on some bits to come in):

     1.   1985 Mopar non-CCC Quadrajet 17085408-1994 (should have been used on a 360 4bbl truck)
           a.   Chevy-style Single Main Air Bleed carb (.080-.090” main air bleed size)
           b.   Mopar throttle linkage primary shaft (required for my application)
           c.   Electric Choke with rear pulloff only
           d.   72 Jets / 54M Rods
           e.   EA Secondary Rods on L Hanger
           f.   Idle Air Bypass ~0.050”
           g.   Carb has clearly been into before, but the work seems quality (bushed primary shaft, threaded APT adjustment plug, brass float, no marred fasteners or other nonsense)
     2.   1977 Olds Quadrajet 17057553-3146 (I believe this came off a 350 olds)
           a.   front inlet dual main air bleed carb (~.070” for upper/lower main air bleeds)
           b.   Hot air choke with dual choke pull-off (adjustable front pull-off) and slotted secondary air flap shaft
           c.   74 Jets / 48M Rods
           d.   CV Secondary Rods on F Hanger
           e.   No idle bypass (passages are present but not drilled through baseplate, I believe this is due to the hot air choke functioning as a form of idle air bypass)
           f.   No indication that the carb has been rebuilt or modified before
           g.   Has “idle-stop” solenoid on front driver’s corner

(Part 1 of 2)

cal30sniper:
After reading Cliff’s book through several times, the Olds carb seems to have a more advantageous air bleed setup and choke pulloff configuration. I NEED the Mopar throttle linkage, but I don’t see any reason why that couldn’t be swapped into the Olds carb when bushing the throttle shaft during rebuild. I’d also want to ditch the hot air choke housing for the electric setup off the Mopar carb.
Alternatively, I suppose I could rebuild the Mopar carb either as-is, or with the Olds Choke pulloffs and linkage swapped over (the Mopar and Olds bodies have identical bosses for choke-pulloff attachment, but the secondary air door shaft would have to be exchanged – easy job). However, I’m still stuck with the single main air bleed configuration with this carb.

Do either of these carbs have a clear-cut advantage for high altitude tuning on a mild engine based on the different air bleed configurations? Would either of these carbs work better with the primary jet and rod selection I have on hand (listed below)? Am I barking up the wrong tree by even considering the throttle shaft or choke pulloff configuration swap between these carbs? Is there anything special I should expect to find in the idle circuit of the Olds carb due to the presence of the idle stop solenoid?

Jets on hand: 68, 70, 72, 73, 74, 75
.026” Tip rods on hand: 43K, 45K, 46K, 50P
.036” Tip rods on hand: 48M, 50M, 54M, 58S

Thanks!

(Part 2 of 2)

Cliff Ruggles:
Use the Dodge unit.  It is the later single MAB design, side inlet, APT, e-choke and already had the correct linkage for what you are doing.

Call me at the shop when you are ready to do the build, I'll put together a custom kit with tuning parts to set it up exactly for what you are doing......Cliff

cal30sniper:
Cliff et al,

I cleaned and started ‘blueprinting’ the 85 Mopar carb this weekend, and started making basic mods. Someone has been into this carb before (it has throttle shaft bushings already installed, blades were already set to 90deg, and it looks like some mods were made to idle calibration). Here’s the measurements I came up with:

Lower idle bleed: .0785”
Upper idle bleed: .070”
Idle Bypass Air: .055”
Main Air Bleed (Single): .085”
Idle channel restriction: .049”
Idle Tubes: .040”
Idle mixture holes: .076” (I opened to .090”)
Accelerator pump discharge holes: .073”
Secondary Tube Restriction: .036” (opened up to .036”, they were very small)
Secondary POE Restriction: .076-.070”

I’ve got a few questions:
1) accelerator pump discharge holes are huge. Is this normal for a SMAB carb, or did somebody open these up? Do I need to size them back down?
2) secondary POE restriction is also quite large. I have other carbs, do I need to pull and swap some in the 0.055 range?
3) idle tubes and idle channels are large relative to the recommendations in your book. Is this normal for an SMAB carb, or has someone opened these up? I’m at 5600’ elevation, and the cam should provide good idle vacuum, so I’d think I need to be on the small side of your book recommendations for the idle circuit.

I’ve since found an unmolested 85 Mopar Q-jet from a van in the local yard (still has the mixture screw caps in place). If this other one has been messed with too much, I can always start over with the OEM one and swap over the nice bushed throttle plate. At the least, I’ll probably pull apart that other carb and see if I get the same strange measurements from it.

old cars:
Accelerator pump discharge holes: .073”

Flip the air horn over and shine a light in to the discharge hole. I believe you will see you measured the outer diameter not the inner diameter. Same with secondary discharge

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