Carb AFR issue.
 

Hi All,

I'm loosing my mind on this one. I am using a 78 vintage 800 cfm Quadrajet on my 67 FB with a 400. At idle I'm running around 13.2 AFR which is fine. As soon as I place the base of the air filter on (it's a RA setup), the AFR jumps to 14.2 - 14.4 and runs rough. I lift the pan of and it goes back to 13.2 The pan is not interfering with anything, the carb is not loose, its a JET stage 2. This can't be normal. Otherwise the car runs great. Any help would be greatly appreciated:usa2:.

my redneck answer is to set the idle at 12.2 with the air cleaner off so that it jumps to 13.2 when you put the filter base back on.

Shouldn't the AFR fall when you put on the filter? As you are restricting / lowering the 02 coming in? So, it fattens the AFR?

I would say that the addition of the pan is changing the way the air is passing over the idle air bleeds.
If you run the motor up to 3000 rpm with the base on and off does the overall mixture stay the same both ways?

Also if you drop on the air filter and lid does that same Mixture change happen?

On any car/ motor you want to only adjust the mixture screws with the whole air cleaner assembly on anyway, if you need to then a flexible long idle adjustment screwdriver is needed,

beats me. i am just saying that since the base leans out 1.0, go 1.0 richer before adding the base to get back where the OP wants to be.

I get you. Just if it drops when on, bump it up 1.0 then let it drop to a good AFR.

His problem is the opposite of what I'd expect. Something's weird there.

This is why the shop manual says to adjust the mixture screws with aircleaner ON.

Let's look at what the carb is.
Its a 1978 q jet. It HAS APT, which IS Adjustable.
adjust carb till it runs great and leave it.
Can't expect to get so called " ideal" AFR with alcohol fuel and carb/ manifold set up vs fuel injection.

BTW 800 cfm QJs are kinda rare, have most of the primary venturi rings cut away, and have a terrible idle on just about anything.

The one on my cousins 71 HO purred like a Kitten, at a 650 rpm idle in gear , and booster rings are not part of the idle metering system!

A 1978 800 cfm Quadrajet is NOT rare, also they have all the booster rings.
In fact, nearly ALL 1975-later non-CCC M-style Quadrajets are of the 800 cfm casting.

On a jet I would look at the power piston setting. Sounds like it might not be quite seated at and idle, and the air cleaner base is changing the air flow enough to move the pistons. Jet probably has a screw adjustment for it. Try turning the power piston screw out a turn. If that doesn’t work might need the next heavier PP spring.

With no offense meant to anyone, I do NOT understand the NEED for the question???

The Pontiac shop manual says to adjust with the air cleaner in place. So why the debate as to why the AFR changes with the addition of the air cleaner? I cannot understand any circumstance where one would run without the air cleaner; so listen to the shop manual. Pontiac had some pretty good engineers!

Jon.

Like the above and two other reply’s here clearly state, some folks are making a mountain out of a mole hill!!!

From my experience using an AFR gauge for 25+ years, the AFR always changes once the air cleaner is in place. How it changes and how much varies with each car and design, but it does change a bit.

Nothing abnormal about that at all.

I did mess with setting some time ago. I will try it. Thanks!

Setting idle air by AFR isn't the correct procedure anyway. You should be setting your idle air with a vacuum gauge.

The first mistake most people make when they get an AFR gauge on their car is to try and chase ideal stoich all over the place from idle to WOT. My car idles very lean, and I get great fuel economy with lean cruise at about 16:1.

Cars with really aggressive cams like more fuel at idle.

The correct answer for your car is whatever air:fuel setting makes the most vacuum. Also, verify your timing. A lot of folks have chased their tail with idle tuning on their carb just to realize their timing was way off. I was once one of those guys.

If the power piston is not seated at idle then you will see small amounts of fuel dribbling out of the booster discharge hole.

If your not seeing that then your issue is a simple adjustment of the mixture screws with the air cleaner on,

I totally agree, it is a nonissue with afr changes with the air cleaner on/off. Just saying if he runs out of adjustment to get rid of the “miss’ with the air cleaner on, look at the PP screw on the JET because they don’t seem to have much for tolerances or high quality parts in that valve. Sometimes it shows up at idle, have had them come totally unscrewed and make super rich.

A vacuum gauge is a handy tool but like the AFR gauge don't get hung up on the numbers. The best idle mixture I find is the strongest most stable idle in gear foot on brake..many times this is a slighty richer mixture then the AFR "should be" or a slightly lower vacuum reading when in park. I always tune my cars so they can sit at a stoplight for a couple minutes with zero issues.