I've quickly come to find that the newer Magnum motor in my wagon needs more fuel than the Quadrajet can give it as-configured. On the old engine (original to the car LA 318) the carb worked great with the APT mixture screw totally bottomed out (the tapered primary rods drop down in the jets as far as possible).
On the new engine, richening the mixture to 2.5 turns up (the base setting according to some lore) made it too lean to drive. 5.5 up made it acceptable for mild acceleration but it still started falling into a hole at throttle openings where the old engine would get going. At that point I started getting diminishing returns on cruise, and power has never really been up to par. The primary power system is an Edelbrockish spring and vacuum piston deal that isn't affected by the mixture adjustment (only bottom of the rod travel).
No, it's not the secondaries dropping open. I've tested it with the air door wired shut and the start of the flat area exhibits well under the throttle opening required to even start cracking the plates. I later saw my new plugs were lean tan-gray too.
Basically the car is driveable with good behavior at normal speeds and flat ground, but it starts becoming obviously down on power the more you open the throttle. WOT acceleration happens, but at an unremarkable pace. Once you're totally out of the idle circuit it's just not able to get enough gas in there. It's not running like it did in the donor truck. Smooth, well-behaved, but the power's not there at all compared to when it was in the original truck.
After I got the carb apart yesterday, I found I have .070 jets and .040 rods with a .026 power tip. There aren't many new rods I can find with .026 tips that are much of a good fit for smaller jets, so I'm going to switch to larger jets that can use the more common .036-tipped rods. My main source for parts is quadrajetparts.com.
My best-guess base setup for available parts is .077 jets with .048/.036 rods. This will get me about 9 percent richer on cruise (adjustable down some) and 9 percent richer on power. If I also buy a pair of .078s that will let me to further richen to 13 percent.
Should that get me in the ballpark, or should I start with .078s for a bigger base jump considering how lean the thing is on power? I did all of my carb-learnin' on my old original motor which was pretty forgiving, so I don't 100% know what I'm doing with this nice newer one.
Not that I know enough about carbs to help you, but feel like some information was left out. What size is the new motor and is it stock? What size quadrajet?
Er yes, sorry. 318 Magnum, stock, stock 67 manifolds. This is a normal 750cfm non-emissions QJ (not the same as a Holley rating). It's a 1976 carb so it uses the later longer metering rods.
First off: A 750 CFM carb on a stock 318 is BIG. Maybe OK for a lot of full throttle running, but tough to tune for typical street driving.
Second: Does the Qjet have an accelerator pump circuit? Your description of the drivability issues sounds just like my experience adapting a 390 Holley to my 215 Buick. The solution in that case was larger pump shooters and a more aggressive pump cam.
Good luck.
Rog
I'd go up 2 sizes on the jets if you can use the same rods. If you have to go to the bigger rods, then I guess you have to figure the sizes that would give you the same baseline and move up 2 from there, or something like that. As you know, a Q-jet is a pretty forgiving carb at high throttle openings, but can drive you around the bend trying to get idle and cruise circuits perfected. I have found that modern gas as formulated for EFI wants a bigger jet with nothing having changed except the gas and the calendar, and 2 sizes has been about right.
Now, the great thing about having the metering rods is as long as you are at least close in jet size, you can make it right with metering rod and power valve spring adjustments.
How about just finding a set of jet drills? You could keep your stock rods, and as long as you don't get impatient, you should be able to sneak up on it pretty nicely.
In reply to boulder_dweeb:
GM didn't have a problem running them on the 305, or the 307, or the 4.3. The Qjet is very tolerant to being oversized.
Guys, Qjet ratings are not like Holley ratings. They use an airflow-dependent spring door li Ignore the rating, it's not my problem with this carb.
I emailed Lars, one of the Qjet gurus from Corvette circles, and he had some carb book nobody else doesn't and determined this carb came with .070 jets and .039 rods originally (not .040). He said it would run fine with that setup. The carb in its original configuration is for a 1976 Cadillac 500, which was rated at 190 horsepower.
On the old engine it did indeed run mighty fine (there's only a 3 percent difference on cruise between those rods, no power change) but on this it ran lean completely untouched engine-to-engine aside from the idle screws. Magnum heads + a stock roller cam flow more air than an LA head.
Initial success!
I ended up switching to .077 jets and .048/.036 rods to get an even 9 percent bump. I switched to the larger jets as there were no rods available that would work with lower 70s jets and change the mixture the way I wanted. I decided to leave the power piston spring alone to avoid complicating testing.
While I had the carb apart, I checked the old float vs the new and found it was a tad fuel logged. The needle was also hanging off of one of the holes in the float arm instead of straight off the end, which can cause it to never quite seat according to my book. I didn't have that problem (no flooding) but now I really understand why average Joes hucked these things instead of learning all their weird bits. I set the float level at 13/16" (book for this carb).
Doug Roe's book spec for the power piston travel (3.175mm) ended up being about 4.5 turns, which confirms my butt-testing of that adjustment earlier. I may try richening it another quarter or half turn after I have everything else dialed in.
As far as driving, I ran out of daylight before I could mess with the full RPM range (1st gear tops out at 60 at 5000rpm and I live in les siouxburbs) or experiment with the secondary door tension, but all of the open throttle flatness is gone. Noticeable torque bump even with 2.91s. Hurray! I'm going to find a place to take this thing to full brap tomorrow now that civil performance isn't worrisome.
