[F-BIRD'88]

I have been tuning and fixing cars long before there was AFR gauges.
Its just another good tool in the tool box.

A trumpet is just an annoying noisy thing until a trumpet player picks it up and plays it.

I can tune my guitar by ear but like to use the Cool LED tuning meter too.

[Catfish11]

The idle is set with a manifold vacuum gauge 14.7:1 or a bit richer (13.5:1)

I have set the idle with a vacuum gauge and tach. I am getting 20 hg at 750RPM with the vacuum advance connected. The gauge is showing 12.5. Is this fine? Any other adjustments in the the idle screws throw off the idle

[ap72]

Yes it is fine. You did it the correct way, you should use the vac gauge NOT the af gauge. The af gauge is useless for idle tuning. You can also tune your accel enrichment with a vac gauge too, and again the af gauge is pretty useless for that.

An af gauge is good for maximizing mpg's at cruise though. It can also give you a little help for wot tuning but max power will not be made at 14.7:1 it will be somewhere between 11.5-13.5:1 depending on the engine.

[F-BIRD'88]

There are many things that effect how a motor idles.
One of them is the fact that when it is idling the throttle is only open a crank and the fuel distribution is not exactly the same to all cylinders.
If the throttle is too far open or too far closed at idle than what is best.
Then the cylinder to cylinder fuel distribution can be less than best possible.
This means 1 or more of the cylinders runs a bit leaner than the others at idle.
So the afr has to be increased, to compensate ( youa re tuning for the worst cylinder. That makes all the others a bit richer at idle than they need to be.

when you run vacuum advance it increase the idle timing. This increases the idle trpm . This can result in the throttles going to closed at idle.

Just for grins: disconnect and plug the vac advance.
The timing should drop a bit, the rpm will drop a bit.
Reset the base timing and reset the idle rpm and idle mix screws
The throttle will now be a bit more open at idle with the same idle speed.
The throttle angle is a bit different now. The fuel distribution may be better now.
Now the 8 cylinder get a more even amount of fuel mix and the enigne can idle at a slightly leaner/cleaner AFR. try it, again using the vacuum gauge, tach and AFR gauge. find best idle quality. Is it leaner now than before.?

Most smog era Qjets are designed to idle with no vacuum advacen active at idle
"ported vac". If when using full idle manifold vacuum to idle with, sometiems the throttle ange is not right at idle.
This will clean up the idle. The plugs wil stay cleaner and last longer
and you won;t polute the air so much.
It is critcal to get the car idleing with the correct throttle angle at idle at the desired idle speed to get the best idle mix and distribution possible.
This is al the more critical on a QJET beace it only idles on the front two barrels.
An they do not discharge in the center of the mainifold.
You wil not get it perfect and will not get the idle AFR to be 14.7:1 but you will get it leaner and cleaner than 12.5:1
Is this carb on a carb adapter??? or some other thing.
What intake manifold?
What is the base timing at idle?. What is the timing when vac advance is used at idle? what cam is in this motor.
Try this ? It makes a difference.
I assume by you getting 20" vac at idle, the cam is near stock.
You may have too much idle timing
too much added idle timing added by active vacuum adance at idle.
its all about finding the sweet spot. Especialy the best throttle angle at idle.
Of cource any all vaccuum leaks or leaky valve guides will throw things off.

anyways when you set the idle with the vacuum gauge and tach for best idle and rpm. You want to then slightly lean both idle screws jsut a hair from that point.
You want about a 20rpm idle drop. This just leans the idle a hair
The enigine idles much cleaner and will not load up the plugs over time.
This is called lean idle drop adjustment.
The AFR will be still a bit richer than 14.7 but a bit leaner than 12.5:1.

Getting this stuff right on the money is how you get the Qjet gas mileage advantage.
I think if you play with it a bit more you can improve the idle.
Qjets are more critical to idle throttle angle setting than some other carbs
and many are designed to idle best with no active vac advance at idle.
(use ported vacuum) to get the best possible throttle angle at idle .

This is one of the ways you separate the men from the boys and get a very sharp tune. start by getting the idle as good as it can be.
What intake manifold?

[F-BIRD'88]

Some times you have to blow out the air bleeds to clear them.
They get gummed up and effect the idle .
Use shop compressed air. Keep you face away when blowing out the air bleeds.
gas may spray. 1 or quick blasts each gets it done.

When zeroing in on the best throttle angle to get the best idle mix to all cylinders possible, you are trading more or less spark advance at idle for more or less throttle opening at idle. You need to find that happy spot.

Things like a leaky power brake booster diaphram can and will mess with things.
or leaky transmission vac modulator. etc.

[Catfish11]

I running a stock engine set up ,low compression early 90's crate engine rebuilt to spec. by a local company with warranty. 36* total timing all in by 3200. With the stock iron intake, and using a egr, no other smog components. Stock exhaust manifolds ,muffler and cat. Dist . is new with 14 initial , 22 mechanical, and 15 advance vacuum. I purchased the quadrajet from SMI in Ca. about 1 year ago. I also run a can of seafoam thru the fuel system every now and then. The carb sits on a phenolic 1" spacer I placed to help keep the carb cool.

[Custom10]

You will need to disable the EGR to get a more accurate reading with your AFR meter.

[Catfish11]

I think you are correct there c-10. I just got back from doing some tuning and i am glad there is a APT adjustment on the carb. It seems that I was pretty close to where I thought I was with tuning by hand. I still think I am kinda urinating in the wind though by not tuning my main jets first . I will start on that next, right now I am running 76 main jets and the biggest being 77.

Back to my current setting for what ever its worth since main jets are not dialed in. I opened up the cruise circuit with the APT screw 2 full turns and got a cruise reading of 12.5 , so I was pretty close on my original setting. I ended up with the APT setting 1 turn CCW (open) from my original setting and have the 15 and 14 bulbs lighting up. I will have to run it again though since the readings got a little erratic , dont know if it was the EGR operating that was throwing off the signal. I am also going to test all my vacuum components AGAIN to make sure there is nothing that is leaking air into the system. I tried to readjust the idle mixture again but the was no change in the idle mixture on the sensor.

[ap72]

Originally Posted by Catfish11

I think you are correct there c-10. I just got back from doing some tuning and i am glad there is a APT adjustment on the carb. It seems that I was pretty close to where I thought I was with tuning by hand. I still think I am kinda urinating in the wind though by not tuning my main jets first . I will start on that next, right now I am running 76 main jets and the biggest being 77.

Back to my current setting for what ever its worth since main jets are not dialed in. I opened up the cruise circuit with the APT screw 2 full turns and got a cruise reading of 12.5 , so I was pretty close on my original setting. I ended up with the APT setting 1 turn CCW (open) from my original setting and have the 15 and 14 bulbs lighting up. I will have to run it again though since the readings got a little erratic , dont know if it was the EGR operating that was throwing off the signal. I am also going to test all my vacuum components AGAIN to make sure there is nothing that is leaking air into the system. I tried to readjust the idle mixture again but the was no change in the idle mixture on the sensor.

Your O2 readings will be a little erratic. None of your cylinders fire exactly the same and the narrow band sensor was meant for a fairly narrow range.