I've run into two different behaviors on two engines lately that have to do with idle mixture.

1. Olds 455 in my H/O. Out of gear idle is too high and jerks car going into reverse. Setting the curb idle in gear with AC on to 600 and AC off to 650 makes the non-stock cam happy. However, it jumps up to 1100 rpm in park or neutral with the same idle solenoid setting. I believe that I have too lean of an idle condition and need to increase the mixture, then back the idle down to where it should be in gear (since a richer mixture will increase the idle speed) and that should cause the idle difference to be less. You seem to end up in a different mixture spot with vacuum gauge reading if you richen it up to where it no longer increases vacuum, then back off, as opposed to leaning it out until the vacuum starts to fall, then going in a bit. Edit: Initial timing is 12, it's about 30 at 2700. I need to figure out if it stops there. The TCS is gone, distributor goes to manifold vacuum.

2. Ford 302. I have found that, like above, I can control engine rpm at idle via mixture and idle setting. I have found that, with mild acceleration, a richer idle mixture causes some light throttle stammer, and leaning it out helps. Lean it out too much, and it seems to have worse heavy throttle response. It's a little autolite 2 barrel, but I am surprised the idle circuit can affect heavy throttle (if, indeed it does). I do agree that it, along with off idle, and some of the primary jets, make up part throttle cruise.

Question 1 is the one I am looking for. 2 is more of a sanity check.

 

look at your spark plugs. If it’s lean the plugs will show it. They will look really clean and the ceramic will have speckles on it.
Id look for a vacuum leak too.maybe around base of carb.

 

Buy a wideband, quit guessing.
The idle circuit is in play even at WOT.
And if the porcelain has speckles on it that’s most likely detonation, not good.

 

Will do.

I have enough cars with carbs that is probably justified. I suppose, with one of those, you tune mixture at idle to stoichiometric? Are the ones put in the tail pipe any good, or should I put bungs / temp pipe section in to get it after the manifold?

 

best wideband on the market is not going to tell you why it’s lean.helluva lot cheaper to just look at a plug. If your timing is where you say it is shouldn’t be that. Even this sorry excuse for gas we run nowadays shouldn’t knock at where your timing is set. If it is lean I would suspect a a restriction in the carb somewhere.

 

I'll do both. The carb is not the right number coded quadrajet. There's also a pinging condition at high rpm and load. I am running better gas, 92 ethanol free over the previous 90, and a different brand, it may have improved out, but, point being, it's got a 350's secondary rods, not a 455's. I do have a correct numbered restoration candidate that I will send off.

 

Is your 72 HO still running factory low compression pistons and what are the cam specs? I'm learning that the two have to work together to build cylinder pressure and if you are using the stock Qjet it may need some modifications to its idle system.

 

Unfortunately, I would have to take the engine apart to know both as I bought it in its current condition, or at least bore scope camera the cylinder and degree the cam. I think, stock pistons, and a somewhat more aggressive cam. It does operate the AC controls, and does have decent power brakes, but it has a Folger's can sized vacuum can hidden in the fender on the way to the booster.

 

I think you’re in the ball park on the fuel. I wasn’t meaning to run down widebands. They’re a great tuning tool. I always start with the basics first and 9 times outta 10 that’s where you find the problem. Good luck! Let me know what u find.

 

The advantage of the wide band is you can see what is happening under differing loads and driving conditions, then tune the whole range in stages.

1. Make sure no vac leaks, float height would be a good idea too. Timing should be mapped & optimized.

2 Set idle mix using gauge 14.0 - 15.0 will work. 14.7 is “real” gas stoic, iirc 14.3 for E10. Don’t fuss too much, long way to go.

3. Tie off secondary air valve with wire where tension spring is, wrap wire around choke housing “legs” (ASSuming Qjet). Just ensure secondary air valve can’t move.

4. Drive it, make WOT romps, see what you have for mixture. This is how you determine primary jet size. Jet up if lean, down if rich.

5. With jet determined and corrected until you get mid/high 12’s for WOT mixture, continue resetting idle (you likely have to anyway)

6. Now for metering rods, this is your cruise circuit, lean cruise needs smaller rod. Rich cruise, bigger rod. I tend to run this a bit lean but anything around stoic will work.

7. Somewhere in all this the primary piston spring needs to be evaluated, put a small screwdriver or such down vent& see if piston is all the way down. Try it with engine off to get the feel. Piston not down at idle needs weaker spring. I do these more by intuition than science but it works. Still adjusting idle as necessary with any changes.

8. Once you’ve got the primary side of the carb to your liking, tear off the chains of the secondaries’ captivity.

9. WOT runs, again looking for mid/high 12’s. Best thing about sec tuning, other than beatin on the car, is no more popping the top off the carb! Lean needs smaller rods, rich needs bigger rods. I usually run “low” letter hangers but any mid range hanger will get you close, say E through K and it’ll work well.

Its a process, with a lot of trial & error. You will probably be popping the top off the carb numerous times. It pays to have a tuning kit on hand or you can order jets, rods, etc as you go but that’s a pain and adds even more time on to the process. If you have other carbs, you might get lucky and be able to use something from them. You can dive deeper into tuning it too, but this will have you running well and you’ll be really good at carb R&I and “top popping.” I can pretty much remove, pop, change jets or rods, reassemble and have it running in about 15 minutes, it’s not that bad.

Realistically this should be done with any and every carb, even built to theoretical specs.

Pro Tip - Coat your base gasket (neatly!) on both sides with anti seize so you can reuse it over & over again before the paper tears.

If I forgot something, misinformed you or was vague/confusing I’m sure someone can correct me, fill blanks or clarify.

Rotsa ruck!

…

 

Good info bccan. I would add a couple of things though.
If you tie off your secondaries, and only run on the primaries, that’s not always a good gauge of where jetting needs to be on that side. Because the primaries are so small, that’ll increase the signal at the booster and actually pull more fuel from there than if the secondaries were open as well.
I just do a light load and heavier load cruise (step on the brake pedal for a second or so) on just the primaries, not WOT.
Then tune the rest, including the secondaries. And you’re right, he’s in for a long day, especially with a Qjet. Change the fuel pressure just a little bit and your fuel curve will change.

 

Excellent advice from all. Next big trip is 7 months away, so this is doable.

Tailpipe or bung sensor?

 

Bung. And AEM 30-0300 wideband kit.

 

Alot of bad reviews for the newer ones. On Jegs, Amazon and a few on Summit. Like a lot of things that are made today.

 

Mine have been flawless. What were the issues?
BUT Holley bought them a couple of years ago so……

 

My from 2018 is working fine. New one's O2 sensor fail. And gauge problems .

 

you don’t tune idle with an O2 sensor. You set idle to give maximum rpm. That’s the correct afr for idle.. there is no such thing as a correct idle afr.

best way to set idle mix is with a digital rpm readout timing lite so you can see exactly what the engine wants.

with the engine hot and idling in gear, adjust both mix screws in or out the same amount to find the max rpm you can get. Then set idle rpm. then check for max idle speed again with mix screws ..

 

I took Mark's advice a while ago and bought a wideband to turn my Qjet after adding a 20-22 cam. Settled on the AEM 30-0300.

It's been very helpful adjusting idle, cruise and WOT. I would highly recommend it!

With respect to idle adjustments for my application it' has helped. Using a digital rpm / timing gun and vacuum gauge I normally see very little change with 1/2 and full turns of the mixture screws but a full turn can result in almost a full point change on the AFR which has helped me get the idle in gear rich enough to where the engine like (always the toughest tune when adding a longer duration cam).

Just my experience.

 

I have a FAST dual O2 wide band that has data logging/down loading, playback, max and min peaks , left&right individual or average, test/run file history and other features.

I do not use it for setting idle afr or RPM . The engine tells you what’s correct by gaining RPM.

I’m not impressed with the AEM. I recently helped a guy set up his carb with his AEM. What a pain in ****. No playback..nothing . The things flashing numbers so fast you can’t remember what’s what. During a full throttle run it was going crazy, we tried to record the read outs with our phones but that doesn’t work because it’s just a blur after.

the only thing the AEM was good for was a steady state RPM afr like cruising ..useless for figuring out afr when accelerating WOT throttle or part throttle through the rpm range..even with two people , one driving and another trying to watch the gauge.

im curious to know how you guys use that thing for anything other than steady state RPM?

 

Having a fast dual O2 sounds like the perfect tool. Not knowing that anything better was available I just worked with what I had. I mounted the gauge on the top surface of the steering wheel column for easy visibility.
As you mentioned steady cruise is the easiest mode to evaluate the appropriate primary rods.
WOT is harder but only because the initial AFR is brief and the AFR number climbs quickly, but catching a glimpse with the gauge mounted high is doable.
With the wideband I was able to select the correct rod/jet combo. Without it I would just be guessing, using spark plug color, gas mileage and whatever other indicators people use.

 

if you’re using the most simplistic gauge out there..why not use analog ? At least it’s not flashing ten times between two full numbers at one decimal place .

this makes more sense than a rapidly flashing number with a decimal point you can’t even see if it’s too fast.

that AEM reminds me of when car companies switched to digital mph Speedo’s in the 80’s..everyone thought that was soo cool..for five minutes. They were crap..drove the driver crazy.

this basic analog makes more sense if you don’t care about play back or any other options.

 

I’ve used those too. But to me I don’t see a difference between digital numbers moving around quickly or a needle that looks spastic.
Just saying.

 

The question was not how to set idle with a tachometer and a vacuum gauge, it was discussing if the idle drop from park to drive can be affected by idle mixture and rpm setting, specifically if the idle is set high and the mixture is lean. I have found that there is a difference, at least with some engines, on whether you iterate positively (increase mixture, back off RPM back to desired idle, repeat until highest vacuum, then back off however much of a turn) vs negatively (have high vacuum, back off mixture, increase idle speed until vacuum drops at given rpm) regarding where you end up at.

 

I'll bump this as I will be getting into it soon.

I've been working too long today, so I'm not thinking right. If you have a quadrajet happily idling away, and you start backing out the mixtures screws, making it richer, the idle will increase, and the vacuum will increase. If you KEEP doing it, you hit a point where the vacuum starts to go down again, right? Unless you have a vacuum leak somewhere.

 

And when you put the air cleaner back on it will enrichen it more.

 

The Ford half of that behavior got solved last fall. There was a vacuum leak. With that plugged the idle mixture screws had more control and the mixture was able to richen to the point of losing vacuum past the peak.

 

Not a tuner or professional by any definition. Can heartily recommend the O2 sensor as a tool in your quadrajet toolbox. I used this one, just to be picky about tuning all 8 cylinders:

https://www.innovatemotorsports.com/...0Gjr24gp91dG1F

I think you’ll get 90% of the benefit from just one side or the other.

Be aware that as a purely mechanical device where vacuum is working against springs, you can expect an error rate where the engine is not running quite at 1.00 lambda or 14.3 A/F ratio. The game is to ge the engine to run right and use the A/F ratio or Lambda as a guide, not a strict rule.

Drive it with the gauge for a few weeks and with a little patience, you’ll be delighted with how nicely you can make a qjet run. Those carbs were amazing for their day.

Chris