Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Frankencarb?
Shiny:
Thank you again.
I will re-visit my primary blade seating and ask for guidance on this If I'm concerned. I can compare the slots on 2 or 3 bases I have laying around to see if this 29240 MD slot design is obviously more liberal than others.
Shiny:
After following Cliff's book to systematically inspect every circuit, I think the carb might be usable in spite of being a rebuilder mash-up.
I found the airhorn has also been swapped. With eBay being my only reference, I think the carb originally had an external vent but now does not.
I am ignorant and inexperienced, but I could find no missing features, no extra features, and all circuits seem to be connected correctly from baseplate to airhorn.
Givan I have no source for "original" measurements, I figured a comparison with Cliff's "Recipe 1" would be the best shot at tuning.
Please look at the attached comparison and tell me if I need to change anything. It is going on a stock (mild) 1970 Pontiac 350, Summit 2800 cam, dual exhaust, 71 intake, 3.55 rear, TH350, at 5800' elevation.
The only "big" difference I see is the main air bleed in the airhorn. Mine is 0.036", Cliff recommends 0.070". Do I need to open this up?
I did find some debris in the idle circuit and improved the primary blade seating. The blades now fully cover the transition slots when closed.
I'd like your recommendations before I call Cliff and order gaskets. If I need to change parts, I'd like to know first.
Thanks for your help,
Mike
Kenth:
Here is the recipe i have found in several 7029240 units, give or take .001" on measurements.
Buick 430
Carb # 7029240
jets 70
prim.rods 45B
idle channels .050
idle tubes .032
upper idle air .054
lower idle air .080
idle air bypass 0
idle discharge .086
main air horn .040
main air body .069
main air nozzle .054
sec rod / hanger AY / N
sec acc./ pri. acc. .054 / .028
sec air tube / Fill .025 / .029
idle screw taper S
sec air valves 1.30
Using this Buick 430 Qjet on a Pontiac 350 w/2800 cam i would start with open the idle tubes to .035"-.036", use .073" main jets and keep the original main airbleeds.
Usually there is no need to enlarge main jets keeping the original main airbleeds, but due to the smaller/shorter, less exposed, brass nozzles in center booster ring larger jets is required to cure off-idle hesitation and keep up fuel delivery for the Pontiac V8.
Idle bypass air is optional, if more than .040" transfer slots are exposed at proper idle speed, you may add .050"-.080".
Hope this makes some sense.
Shiny:
Hi Kenth,
Yes, it makes sense and I greatly appreciate your help.
It seems most of my carb is "as new", which is good news for me since the airhorn is not "correct".
Please forgive my inexperience, but:
- My idle tubes are 0.037". Is this too much? Worth dialing back to 0.035"-0.036"?
- My main air restriction is 0.036". Worth drilling to 0.040"?
- My jets are 70, rods are 44B. Worth changing to 73/45B given I live at 5800' altitude?
- My secondary rods/hangers are CH/O. Worth changing to AY/N? CH is short, AY is long. Is altitude a consideration?
I think I'm close to feeling OK about this confusion, thanks to you and Cliff.
Mike
Kenth:
In 1972-74 Pontiac used some Q-jets for "Altitude" usage.
F.e. i have found the main difference between a "Regular" and an "Altitude" 1974 unit is a weaker power piston spring, 7037851 (yellow) in the Alt. units instead of the Reg. 7029529 (purple) and a couple of thousands larger idle tubes, that´s it.
If you put in a weak power piston spring like the 7037851 in your carb as is i think it would work just fine at your altitude.
HTH
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