I've had a 1976 500in Cadillac-spec Quadrajet on my car (1967 Dodge Coronet wagon) for the last 3 years. I got it for free, it was a supposed off-the-shelf rebuild that never got used. It ran well on the car's original 318 and got me 16-18 mpg on the highway. The early 70s Carter AFV I had on there before would only get me 14 at best even after jetting down from 440 spec.
It would pull nice and smooth to 5000rpm with pretty fun power for a worn-out LA 318 on 2.91 gears. It would get on the Summit wiener cam past 3000 and really make a good old show until 5000 (it'd keep going but I didn't want to break it, at least not that way). I sang the praises of Quadrajet! Always starts, runs and guns, miserly for being on a two-tonner. 2-1 kickdowns were proper silly for this old thing.
Since then, I've swapped in a stock 318 Magnum from a 96 Jeep GC. Right now it's all-stock, original 67 manifolds and a cheapo dual-plane intake. Stock roller cam, duration at .050 is 193 Ex/184 Int. with 112 degrees separation, stock heads. Great even compression, still has the factory crosshatching on the cylinders, ran great in the donor truck, yada yada. Wonderful boon for basic driveability - it's happy to be in gear and drive away in a tenth of the warm up time of the old engine.
It starts and idles great, and I've been able to dial in the APT to give it some decent scoot off the line, but it's acting like it's getting lean outside of anything above mildly-cautious-dad-driving throttle openings. Again, this carb is for a '76 Cadillac, so I'm assuming that's a possibility (I don't know how emissions-choked Caddy 500s were in the 70s but I assume a fair bit).
Fortunately since this is a 76 carb, I was able to pop the plug on the air horn and tap a hole for on-car adjustments. I ran this carb with the APT pretty much cranked all the way down on the old engine because it ran no different higher up. On the new engine, if I start off with the APT 2.5 turns up from the bottom or lower, it falls into a lean condition if you barely open the throttle past gentle acceleration.
Raising it to 5-5.5 turns improves it a whole lot and gives it some pretty decent scoot, but it's still only really happy at "normal driving" throttle openings. I tried backing it off gradually by half turns as I drove and around 5.5 turns is where I was into diminishing returns.
All of this testing was done with the fuel pressure regulated to 4.5 psi with a usual basic Holley regulator. I'm sure it's not the secondaries dumping open too early. I butt-learned that by driving it with the door wired shut, the door slightly past zero tension and with the spring tension re-set to be slightly too strong to make sure I wasn't confusing a secondary bog.
After I was done rechecking the ignition timing (usual 11 initial/34 all in 2800-3000) I pulled the carb off to take it apart and see what I was dealing with. I never had a reason to mess with it on the old boat anchor.
The carb (# 17056230) has .070 jets and rods that are stamped "40L," which I can't find any info about but they measure .040"/.026" tip. I was surprised to find that one of the rod tips was bent two different ways (see attachment), which wasn't enough to hang up inside of the jet (I've checked the power piston for engine-off free travel and on-vacuum seated travel and it didn't get hung up) but I imagine messed up things for one half of the engine a little.
Since it will eventually matter, I have an R secondary rod hanger and DH .0567 secondary rods. In the old engine it would show a little bit of greyish smoke at the very top end but who knows if that was fuel or the remaining life-force of the engine.
I've got Cliff's book and Doug Roe's book as well, but I am still a real greenhorn with this stuff. I've owned this car for about 3 and a half years and it's the only carbureted car I've ever owned - I'm 30, my other cars are a NA Miata and a couple of boring Subarus. I know a little about Carter BBDs and AFVs, a Thermoquad we will never speak of again, and this Quadrajet that I never really dug into because the original engine ran so much better than any other carb before.
In continuation of my self-discovering quest into the iron of old, I hope I can beseech you wiser men to advise if I ought to change either of my jet/rod or jet/hanger combinations, or possibly try a different power spring piston?
If I can tune with confidence in my jet/rod/hanger combo it will save me a lot of... bad feelings. I don't want to find out my best educated guess at switching to .71 jets or the strongest power piston is halfway toward a solution and halfway into a different problem. I really want to unlock the potential of this carb on this motor.
