lars

As promised in another topic post here on the Forum, here are some photos and a run-down on the rebuilding and testing of Jeff_Keryk’s Q-Jet.

As noted in other posts, the Q-Jet tends to get a bad rap for poor performance and drivability issues. The problem is that there are no new Q-Jets available from any source – all Q-Jets out there are either 35 – 45 year old survivors, or “rebuilt” carbs that have been hacked up by a multitude of “experts” over the years. This results in some pretty bad running carbs that live up to their “Quadra-Crap” nicknames (even though a badly set up, run-down, 45 year old Holley would run as poorly).

Here is a good example of the condition of the carbs I’m getting in for rebuild now (this is not Jeff’s carb). This is a 1971 Q-Jet in the “as-provided” condition:


A little work, and the carb is now an excellent-running carb that will provide years of trouble-free service. This is the “after” shot of the same carb:


So Jeff sent me a 17059222 (1979 Chevy 305 Malibu) Q-Jet. It had been purchased from a “good” carb shop, but had some drivability issues on Jeff’s ’68 427/390-horse Vette. The throttle seemed to “stick” after warm-up, and it didn’t run well.

A teardown showed a few problems:
The original ’79 carb was jetted with 71 jets, 40 primary rods, and “CH” secondary rods. On an emissions-loaded 305, this carb was set up very lean, so we know this jetting baseline will be too lean for the 427/390 in Jeff’s ’68. The jet/rod combination of the stock carb produces a jet metering area of 2.7 thousands of a square inch, and we want something in the 2.8 to 2.9-range for the 427. The carb builder had set this carb up with a 75/49/AX combination. The 75/49 combo produces a metering area of 2.53, which is 7% leaner than the stock lean setup..! At WOT, the AX secondary rods and 75 jets would make this carb go pig-rich, so the setup was just plain bad overall. To further aggravate the problem, the APT (adjustable part throttle adjustment of the power piston) was set full lean, adversely affecting both idle and part throttle cruise. So how did this carb even run..?

Tearing the carb down further showed that the needle clip was installed backwards in the float arm, partially jamming the needle in the seat. But the interesting point came when the electric choke was removed: The carb had been converted to an electric choke system, but the builder had failed to removed the hot air choke cover gasket from the choke housing. The electric choke gets its grounding connection through the choke housing, so electric chokes do not use the cover gasket. By installing the cover gasket, the electric choke was completely inoperable, and stayed fully closed at all times. To correct for this, the choke pulloff had been adjusted to pull the choke open quite a bit upon start, but the choke remained “on” at all times. This rich condition produced by the partially closed choke allowed the carb to idle and operate in spite of the extreme lean conditions produced by the jetting setup.

Other issues found:

• The secondary airvalves were misaligned on their shaft, jamming the blades into the airhorn casting and locking the airvalve closed.
• Throttle plate screws were completely loose, allowing a vacuum leak at the throttle plate.
• The inlet filter spring was missing, allowing debris to enter the carb.

It should not be surprising that a carb set up like this would give the Q-Jet a bad rap… So to fix it, the carb was given a new jetting setup with a 74/41/CH combo. This produces a jet metering area of 2.9, yet provides good power enrichment at WOT. The CH secondary rods are better suited to the ’79 carb, and will produce very nice full-throttle performance. The APT was raised to the correct position, the secondary airvalve was repaired and correctly adjusted, and the float system was set up to the right height with the parts and components correctly installed.

Here’s the work area with engines, carbs, and distributors in process:


In addition to Jeff’s carb, there are distributors and Holley carbs in the works, too:


Newly arrived carbs and distributors waiting for evaluation anid staging in the process:


Here is Jeff’s completed carb ready for testing:


The test engine is conveniently located next to the bar:


Jeff’s carb on the engine just after completing the 20-minute test run – note choke is now operating and wide open after the hot-run:


Choke wire can be seen hooked up after the run:


I set the rpm to just over 800 on my XE268H-cammed test engine. This produces a pretty good idle on most performance engines:


Air/Fuel ratio is set and verified with a wideband. Modern pump gas likes a ratio in the low 14’s, and Jeff’s carb is set up just perfectly:


Carb is now running extremely well. Throttle response is instant and fantastic, fast idle is working, and the choke is opening up as it should to produce a stable hot idle speed with perfect mixtures. This carb should run very nicely on his 427 finally!

Lars

v2racing

Nice work Lars!

It is amazing how badly people can mess up carbs!

rcread

Please don't post these pictures. I think people have surgery on dirtier tables than your workbench. Now I'm going to have to spend my afternoon in the garage cleaning and organizing. I do have beer out there, so maybe it won't be so bad.

I actually do enjoy reading about these projects and the real world problems faced rebuilding an old Q-Jet.

Jeff_Keryk

Wait a minute... Why is that choke in the vertical position?
It has always been open just a hair...
My old L36 won't know how to act.

Superman (Lars) to the rescue!
If Lars threw footballs for a living, he would be Colin Kaepernick...
Cool!

rogman16

Great job on the pics and explanations... Really love reading your diagnoses... It's amazing that with all the bubba repairs that some of these carbs can still run!!!

Love seeing the queue of boxes with Carbs waiting for their turn on the massage table!!!

Thanks for restoring another Q-Jet!!!

Rogman

lars

Thanks for all the fun comments! Yes, it is amazing how bad a carb can be set up and still run reasonably well... we see the same thing on the dyno, too: A carb can be off on its jetting, producing an air/fuel ratio one to two points off optimum, yet only be off on peak power by 5 horsepower. Bigger gains are usually made on optimizing timing (both ignition and cam) rather than flogging the carb down to single-jet size changes...

...and for my mid-day project, after having finished Jeff's carb, I just finished putting together the blower engine that was seen partially assembled in one of the photos above. This one put out 435 horsepower normally aspirated. Since that wasn't enough, I've now dropped compression ratio (piston change) from 10.5:1 to 9.3:1, swapped cams to something more suitable for the blower, and done a custom Vortec installation that will actually fit under the hood of a C3. This will be the next dyno project, so stay tuned for the numbers results and photos from this one. Notice that I don't run a Q-Jet on everything, in spite of what some people comment about me. I even have a Demon on my '64... (the mechanical pump shown on the engine will be used for the initial run-stand test only - it is not the "real" fuel system installed in the vehicle to support the 10-pound boost):





Lars

Blue73Shark

Another great course from the University of Lars. Really appreciate the education you provide us - thanks.
Fran

Shark Racer

I almost missed the note - I skipped to the pictures and got confused when I saw a 4M and then an M4M.

Is the 14.1 a cruise AFR or an idle AFR? I'm finding on my engine that when my idle starts to exceed about 13.5 it becomes erratic. (Maybe due to low static CR?)

I've found on most of the carbs I see that the choke system is usually pretty messed up, float is maladjusted (exceedingly high or exceedingly low, never in the ball park), and that primary metering rods tend to be a complete crapshoot.

Capt. Shark

My Tuned by Lars Q-Jet is still kickin' *** and takin' names

drwet

Awesome post. Thanks for the education. One question. If I understand correctly, you are suggesting that the 427 requires a richer setup than a 350. Why? The carb doesn't know what size engine its on. It just sees airflow and air fuel ratio.

Shark Racer

The factory calibrations were for emissions. There are also systems in place to mitigate the high operating temperatures (and NOx) that come with the leaner cruise calibrations, that one would not find on an earlier car.

The ratios are not beneficial to throttle response or longevity; even the 305 would benefit from an increased ratio.

The engine does "sense" engine size to some degree via the air-valve mechanism, as well as through the various metering circuits.

lars

That's correct, and I didn't mean to imply that the carb needed to be richer beacuse it was a 427. My implication was that a non-emissions performance engine (Jeff's 427) needs to be richer than an emissions-legal 305. The displacement is, as you note, fairly irrelevant, since a carb does not know what size engine it is on - it meters fuel based on mass airflow through the venturi.

Lars

427Hotrod

Good stuff as always Lars!

JIM

RATT7

Thanks for the great write up again lars !

cardo0

Thanks for sharing your results. The Qjet can be quite a project to learn carburetors with lars. Its cool to see what can be done with a seasoned carb - helps encourage us. Some of those details are only learned with years and years of experience. It would be nice to learn some of your diagnostic tricks too.

BTW let me know if u need any Qjet cores. Lately i've been collecting the 800cfm qjets with chevy inlet and electric choke of course.

cardo0

Jeff_Keryk

Update - I received my FI Unit, I mean Q-Jet.
I will try to find some time to install soon and post results.
And I can report that Lars missed his calling.
He packaged the carb so well, he could have done very well in a shipping and receiving department!
I hear Wally World is hiring...

Jeff_Keryk

Reinstalled the Q-Jet this morning. Poured a little gas down the bowl vent tube. Fired immediately and fast idled @1400 RPM. Sounded just right. After maybe 2 minutes, I kissed the throttle; the now-working choke was opening and the idle dropped a little. Another minute, and the idle dropped to 650 and the choke blade was vertical. I adjusted the idle speed about 2/3 turn; idle was ROCK STEADY @ 750. Took her down the street. One word: Torque MONSTER. I don't get on it in neighborhoods, but I can say from off-idle to maybe 4K (where I let off) this thing was all break-away; it wanted to pull hard and run. It wants to get side-ways with the fat sticky Comp TA's. The sound? Pure heaven. And I was just entering the power band...
Lars: Using your knowledge, skills and considerable abilities, you just made my old, Plain Jane L36 Roadster come alive. This piece of Americana lives to make noise for another day.
Thanks Lars. But you just might break the old Muncie...
For those of you who know there is more in your motor, get a Q-Jet. You won't be sorry. I will post pictures and more soon.

lars

Sounds like I happened to get lucky on the carb setup...
Glad to hear it's running as it should, and it's great to see you're having some fun with it - good job on the install and setup, and don't stop spinning those tires..!

Lars

vetteman1976

awesome write up!! I know who I'll be kindly asking for assistance on my carb rebuild in the near future for sure...
j

Jeff_Keryk

President Thomas Jefferson:
"I'm a great believer in luck, and I find the harder I work the more I have of it."

Jeff Keryk:
"Lucky my ***..."