Technical Tuning Quadrajet help needed

This morning I rebuilt the q-jet on my (as far as I know stock) 283. A couple months back I "mondernized" it by adding a dual lambda and discovered that at idle I was running ~10:1 on both sides. Cruising I was at 16-17:1 and WOT shot up to 20:1 the only time I did it. Since then it's been sitting in the garage waiting for a cool enough day to sit out there and work on it.

Today I tore it down and replaced the gaskets, needle and seat, and float.

I discovered the primaries are 0.073 jets with 48B metering rods, and CT secondary rods.

For background the truck had been living in Wyoming (5000ft elevation) and is now in Houston texas (30ft elevation).

I installed the idle screws at 3.5 turns back from bottom. Took them both in about 3/4 turn to get it to idle and had to dial up the idle speed a little (1/2 turn). But the AFR was running at 20:1 on both sides so I backed it into the garage and turned it off.

Now I need your help with what I should do next. I'm not much of a carb guy but I'd hoped cleaning it up would help and instead I made it worse!

Also: I have a new Wix fuel filter showing up any minute at O'Rielly as I know the current internal filter is >10 years old.

Should I get new jets/rods on order? If so what size would you all recommend?

Is there something else I can try or look at to get it down into a safe AFR?

 

Carburetion 101 - "A carburetor meters and mixes fuel AND air"!

The exact same jet rod combination can meter VERY differently in different carburetors depending on the air jets. Would be helpful to know which Q-Jet you have.

As far as the idle screws:

Generally, idle screws on pre-1968 carburetors have a range of 0 - 1.5 turns. Federal smog emission began in 1968. To more finely tune idle mixtures, taper on idle screws was changed from the short tape to long taper. Generally, 1968 and newer carburetors have idle mixture ranges of 1 to 3.5 turns.

A/F meters are great, as long as the user is CERTAIN the meter's calibration is correct. The leanest carbs I can remember were the Chrysler lean-burns which were calibrated at 18.2:1 AND REQUIRED HEAT TO RUN. 20:1 isn't going to run!

Jon.

 

Hey Jon,

Thanks for the quick reply.

The carb number is 7042240, which I believe makes it a 68-72 Buick model?

You're right on the AFR meter, but being brand new I'm pretty confident in the calibration. I didn't cheap out on it so I'm pretty confident in the Bosch O2 sensors and the Innovative meter...

How was AFR handled pre-O2 sensor? I know I can adjust idle mixture to get maximum vacuum. But that wouldn't necessarily mean I'm running a good ratio.

 

Did you mess with the idle orifice tubes? I had trouble with a Qjet idling in gear, and acted as if it was not getting enough gas. I removed the idle tubes, and with pin vice drill bits found what diameter the orifice was and went .oo2 larger, and helped a bunch with idle in gear. They can be a pain to remove, but with some patience they will come out.

 

It started at 10:1 idle AFR, then after you went through the carb and played with the idle mixture adjustment, it ended up at 20:1. I would expect it to want to be around 10 to 13:1 to idle with decent idle quality and not have any off-idle transition problems. At cruise it should be 13 to 16:1, and under WOT should be around 11 to 13:1.

The high AFT at WOT might be due to the secondary air valve spring being adjusted a tad loose.

But the most important thing to remember when messing with carbs with an AFR meter, is that the way the car runs is way more important the the numbers on the meter! This is a difficult concept to grasp. Carbs are always a compromise, you won't get EFI numbers unless you're a magician (like gimpyshotrods, for example). Although you should be able to get closer with a Qjet than with a Holley.

What you need to do is to play with it, and see if you can get it to drive ok. Use the AFR numbers as clues as to whether it's rich or lean under certain conditions, but don't have perfect numbers as your goal. Instead, have a nice smooth idle with little smoke and no hesitation, smooth cruising, and responsive WOT acceleration as your goals.

Spend time, experiment, see what it likes and does not like. And don't be surprised if you find that you might want to get another carb(s) to play with, as sometimes Qjets have been screwed up by previous owners/mechanics/rebuilders in the past 50 years.

 

When it was measuring 10:1 at so called idle, any idea what the plugs looked like? Black at all?

 

@big duece I cleaned all the times while rebuilding it. Ran wedding to cleaners through everything. But nothing beyond that for working on them.

@squirrel thanks for the insight. I've always been an EFI guy so this a challenge for me!

But considering it idled at 10:1 before I should be able to get it there again...I hope

I don't have the ability to tune part throttle, so that's where jets and rods matter right?

 

Yes, that's where they matter. But they seldom need to be changed.

What did you use for the float level setting?

 

I set my calipers at 0.125" and checked that the float was there off the secondary well. That's what the video I used as a reference said was the place to be.

 

Was that video for the same carb model number as the carb you have? There were a lot of varieties of Qjets made over the years, and a whole bunch of different specs for float level, etc. Make sure you set it up to the specs for what it is.

 

Perhaps not... I've got a reference booklet around somewhere.

 

If it cruises at 16-1 and doesn't surge, misfire, or run hot, that's what you want. What you don't want though is 20-1 under power or acceleration. That's going to roast an exhaust valve. Somethin' ain't right, Gus.

About 12.5 or 13-1 is right in there for safe rich WOT. Remember any exhaust leaks in the manifold will cause a false lean indication, and so will ignition misfire.

O2 sensors measure just that, O2, so if there is unburned fuel due to misfire, there is also unburned O2, and gauge will indicate lean. It sounds strange at first but that's how it works.

To keep plugs from loading up it's better to have the idle mixture carefully set. I don't bother with the AFR meter on that, like Squirrel said, set it up so it runs good versus chasing numbers to three decimal places. It doesn't take much to foul plugs if they are set wrong though.

 

Exhaust leaks before the o2 sensor will give you a false lean reading.

Squirrel is right when he says that the way the car runs is way more important the the numbers on the meter. Besides the way it ran I use to go by spark plug color, vacuum gauges and exhaust sound and smell. I still do.

 

Can't help with AFR numbers, etc., but .125 at lowest point is probably too high. ....187 range probably better for a non- race car.

 

Seeing 283's didn't come with Qjets..I'd say .073 jets are too big...069 to .071 would be better, IMO

 

Agreed. I ran .071 - .073 on my non stock 400 sbc. You might consider smaller. I want to say my 69 Q Jet came with .066 stock but I can’t remember. Then again gas is much different now.


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283's came with WCFB's a whole different carb than a QJ

 

Rochester 7042240 is for a 1972 Buick 455.

The calibrations for Chevrolet (even a 454) are quite different from those for Buick. Not just gasoline jets and rods, but also air jets. Also, the placement of air jets for Buick is significantly different from those for Chevrolet. Generally, the engineers that designed these things made changes like this for a reason.

I would suggest the easiest method of tuning would be to simply find a Q-Jet from a Chevrolet 350 to start with.

Check the second line in my signature block.

Jon.

 

Thanks guys!

To clear a couple things up:

- My dad put this truck together from a variety (obviously) of sources. It's been running "ok" for years. But was rarely driven so he didn't have a lot of impetus to have his shop guys work on it. (and personal projects for the owner don't make a shop much money). He's still around so we talk, but he isn't a mechanic and is 1000 miles away.

- I started the carb rebuild mostly because I did have drivability problems.
-- Hard start, had to pump the pedal and crank it repeatedly to get it to start. Then had to feather it until it was warm enough to idle on its own. (Foolish me planned the rebuild so I didn't just troubleshoot that)
-- Cruise was fine. Would idle a little weird at lights but then would drive off without any major problems as long as I didn't stomp on it.
-- WOT / heavy acceleration I would get stumbling. Have to back off on the throttle and then gently wrap it up. But once I had the AFR gauge it felt dangerously lean.

With that, I would like to eventually drop a Holley Sniper EFI kit on there and call it done. But life doesn't currently think that should be my financial priority. I'd like to get this q-jet back into working order so I can safely cruise around. The reference manual I have only lists q-jet model numbers that start with "1708". Can anyone point me in the right direction for the correct adjustment specs? I'm headed back out into the world of Google to try to find it myself now

Edit 1:
I found a 9D-SS manual (https://www.google.com/search?q=del...rome..69i57.3495j1j7&sourceid=chrome&ie=UTF-8) that has my carb on page 35. Guess I'll be taking that bad boy back off and adjusting the float correctly. Then dialing in those idle screws from where they are.
Thanks everyone for the help! Hopefully I can get it running again today...

 

As I said earlier, be sure to check the secondary air valve spring adjustment. If it's a bit loose, the air valve will open with too little air flow at WOT. This results in a lean condition, because without enough airflow, the fuel will not begin to flow on time.

The cold start is mostly dealt with by the choke mechanism. If it's not opening, the engine will blurble and emit black smoke (and have really low AFR). If it's too far open, it will sneeze and want to die (and have really high AFR). The choke is complicated, and my guess is the 72 Buick carb does not have the correct choke to work with the Chevy intake manifold. That's yet another reason to try a different carb that is designed to work with the engine you have.

It sounds like you are only having problems with things that there are adjustments for, so there is hope that you can get it driveable. Be patient, keep asking questions, etc.

 

Your cold start/ drivability problems sound like classic automatic choke problems .you might be able to rig an electric choke if the vacuum/ manifold heat stove( whatever is on there) isn't working. The hesitation sounds like either accelerator pump problems that, the qjet is known for , or like squirrel said, mal adjusted secondary flap ...... Lotta guys loosened the spring up so they'd make that vroooom noise ......

 

Update for you guys. I pulled the carb, adjusted the float to 13/32 per the tech spec I found in the Delco documents.

While re-installing it I realized that the original gasket (thin metal thing) covered an unused port on the top of the intake. The new gasket didn't cover it. So I went back to the original. Also dialed the idle screw to 2-turns.

The truck fired up almost immediately and idled nicely. I let it warm up and adjusted the vacuum. Pulling almost 19inHg after dialing it in. The AFR is idling at 18:1-ish.

Now, however it dies when I put it in gear. I did get it around the block by dropping it to neutral at the stop signs.

Would adjusting the idle speed up take care of that dying problem? I didn't do any meaningful acceleration so I'm not sure if it's still stumbling or not (I like my neighbors too much to tear around our streets).

 

See if 2.5 turns richens up your air/fuel and stalling in gear.

 

Since that carb is for a 455, and you have a 283, your idle air velocity is probably lower than what the idle circuit is expecting. Try giving the idle mixture screws maybe 2 1/2 to 3 turns total.

You could also adjust the idle RPM by the use of the throttle positioner screw.

EDIT - Bigdeuce, you type faster than I.

Jon.

 

I can honestly say I'm no longer sure where the idle mixture screws are as I adjusted them to get maximum vacuum. Is it ok having less vacuum to richen up the mixture? (seems having a working in-gear idle would be worth it!)

Also, I don't have a tach so I'm not sure exactly where it's idling now, but is there a consequence (aside from fuel use and noise) to having a higher idle?

Also, after doing this I realize how not ideal this big 800cfm q-jet is for my little 283, I know I won't get it running perfectly. But I appreciate everyone's help in my getting it running well enough.

 

Turn the idle mixtures back to lightly seated and count revolutions just for reference.

 

You might look for a qjet for a 305 from late 70s, I believe the computer control Qjets started in early 80s.

 

The Quadrajets are all basically the same size, and the most magical thing about them is that they will work on any size engine from around 200 CI to 500 CI. The secondary side is huge, and the air valve will only let it flow as much air as the engine needs. It's not too big of a carb for a 283.

Play with the idle a lot, both mixture and RPM, and expect it to take several tries before you're happy with it. Go from way lean (like it is now) to way rich (if you can get it to be way rich), and see where it works best, from a driveability point of view.

Idle speed wants to be slow enough that it won't creep much in gear (if automatic trans) or be running too fast (if manual trans). But it needs to be fast enough that it won't die at stops. I find that the idle mixture needs to be richer than you probably think it would, to keep it from dying at idle. Hence my earlier AFR suggestions.

 

I'm curious about the base gasket(s) some factory qjet setups used 2 composition gaskets with a metal heat deflector plate between them , pics of your manifold and the bottom of the carb would certainly help ....

 

@squirrel I'm going to keep messing with it this week between rain storms. The truck never got taken out in any form of precipitation so I'm still a little finicky about driving it in rain. Knowing I only have 2 (well 3 screws) to play with makes it easier.

@2OLD2FAST I'll snap a photo of it next time I'm in the garage. But on the front side of the intake manifold there is a "trough" with a hole on the right and left side. The left (drivers) side doesn't line up with anything and the metal plate covers it. And thus my ignorance is showing!