mobird

Hey guys,

Just finished up my top end build on my 79 L48. I went from the stock iron heads, intake, and stock cam to Brodix IK200 aluminum heads, Lunati Voodoo 268 cam, and Performer RPM intake.

I have an AEM wideband 02 sensor installed in my exhaust. Before the build. I would hit around 12.8:1 A/F ratio under wide open throttle. After installing all the new parts, my wideband 02 is saying I"m between 10.5:1 and 11:1 A/F (WAAAAAY Rich).

Any ideas why this is? I thought if anything it would run lean after adding much higher flowing parts. I'm currently on 74 main jets in my quadrajet, so I may go ahead and order something like 72 jets.

cagotzmann

Run this test with your unit. It may explain some things to you.

Disconnect 1 spark plug and see if the AFR goes lean or rich.

If it works similar to mine you will notice a more lean condition. What I would take from this is your original setup you may have had a ignition miss, valve problem etc giving you a false AFR reading. Its now working properly with no miss and getting a true reading of the engine running.

REELAV8R

Give the engine what it wants. Any supporting evidence that it is rich? Black exhaust tips. Black ring on the plug(s).
With a performance enhancing move such as what you did, you cannot expect that the jetting or timing will want to be the same as stock.

cagotzmann

He provided AFR Readings. Is this supporting evidence ?

Plugs should verify the AFR readings ?

mobird

Thanks for the promp replies. Ill pull a plug when i get home in a couple hours to verify that its actually running that rich. Ill report back with what i find!

mobird

Ok so I pulled a plug and sure enough it's running extremely rich, plug was black and sooty.

I found some 72 jets in my garage (two sizes smaller) and installed them and it made no difference, still extremely rich.

I pulled the top off my quadrajet again to check float level and noticed that the two areas circled in red in the picture below are full of fuel. Is that right? Seems wrong for some reason.




Also, I have a float level of ~.24" (1/4"). Should I lower my float to lean it out a little or does that sound about right? I've found a lot of different reccomendations on float level.

cagotzmann

I am not a quadrajet guy but Lars would be the one to get advise from for this. I believe you should have fuel in the camber for the accelerator pump.

What does your afr gauge read when you run the engine will a plug wire disconnected ?

Not sure on how you lean out a quadrajet. I believe they use metering rods. Is this what you changed and called jets ?

mobird

I didn't try running the engine with a plug disconnected, but I can.


The quadrajet uses a combination of metering rods and jets. Since I'm currently working on tuning the WOT and all metering rods have the same tip size, they shouldn't affect it.

Jebbysan

I could be wrong....but with the lower vacuum the metering rods step up is going to change......maybe they are not pulling in the hole......
This happens on Edelbrocks/Carters.......works on the same principle.
There is an adjustment....but as I am not a Q-Jet tuning expert.....I cannot tell you where to set it. There use to be some adjustment settings in the old carb kits.....different years and models.....but I do not believe that GM ran Q-Jets on any Chevy engine that was radical......but they did on Olds, Buicks and Pontiacs......so if it were me....I would start by reviewing the settings and jetting on these models....(W-30-W-31, Stage I, Ram Air IV)........as well as the step up spring tension......
Lars may pop in here and tell me I am all wrong......who knows......

Jebby

REELAV8R

I think Jebby is on the right track, at least for the primaries. Where your metering rod assembly slips into the hole in the middle, there is a spring inside that hole. That spring may need to be a weaker one as now you have lower vacuum. Vacuum is what holds the primary metering rods in the jets. Make sure the rods are actually in the jets when you assemble and that the tips are not broken off or bent.
Once you go WOT though things change. The vacuum is gone essentially and those metering rods pop out of the jet via the spring pressure and now the size of the jets is the primary restriction to fuel flow. The primaries are the main fuel supply initially soon followed by the secondaries as the air flap opens.
The secondary side you have the air flaps that begin to open once the volume of flow is sufficient to open them. Mounted on the airflaps is the metering rods for the secondary side. They slide into two brass orficies in the bottom of the bowl. Make sure the orfices are still there. The rate at which the airflap opens is governed by spring pressure on the passenger side of the carb. Initially the spring can be set to about 7/8 of a full turn and adjusted from there depending on whether it bogs or not. In your case you may need to tighten the spring depending on where it is set now.
My primaries are 68's right now. However I am at 4000 feet so they may be somewhat smaller than you will need.

Lars is really the expert. I'm just a hobby shade tree mechanic.
You can also buy Cliff Ruggles' book to learn more about the q-jet and tuning it.

As far as the two holes with fuel in them. The black one is a spacer? I believe and If I recall has fuel in it when I take my carb top off as well.
The other hole is your accelerator pump well and should always have fuel in it.

Shark Racer

mobird, it looks to me like your float is stuck open.

See pic below, this is what the float should look like with no gas in the bowl:



Did you put the retaining clip on the needle valve? You may have installed it incorrectly, which would cause the needle to hang open on the seat.

You can see a photo of the correct relationship on the retaining wire in this photo, it should be perpendicular to the width of the float arm as it travels over the needle valve (NOT through one of the holes).

mobird

In the picture I uploaded the float bowl is full of fuel so that's why it's in the raised position. But I'll double check the float operation when I get a chance, thanks.

REELAV8R

On most carbs I have worked on the proper fuel level is when the float is level. Not tilted up or tilted down. The q-jet seems no different.

Jack605

Had a similar problem with my Edelbrock Performer had to increase the metering rod springs this was due to the performance cam.

Jack

Shark Racer

The performance Q-Jet builders recommend a 1/4" float height, I believe factory spec for a Q-Jet on a late 70's Corvette (which, at the least is what this carb is as it has APT) is lower, somewhere between 3/8 and 7/16 (don't have a spec on hand, it's in Lars's paper and comes in every rebuild kit).

I ran 1/4" on mine with no problems.

Some needle seats do have sealing issues, I've heard.

bluedawg

What are your timing specs? Duration needs timing to burn the fuel. It might be set right..

mobird

I have it set at 35 degrees total timing. I haven't run the engine much since it's running so rich. Just cam break in and some short cruises down the street to test my A/F ratio. So all I did was disconnect my vacuum advance, crank the engine up to 3000 RPM, set my dial-back timing light to 35 degrees, and adjust the distributor.


Does that sound decent? I figured 35 degrees should be close enough for now.

bluedawg

q

Thirty five sounds about right, what are your cam specs and what is the initial timing set at? The more fuel that is drawn in at idle will need to be lit sooner so that it has time to burn. If not it will black smoke and burn your eyes ect.

mobird

Not sure what the initial is, I just let that fall where it will when I set my total timing. I can check it sometime this week though.


Cam spec card: http://www.lunatipower.com/CamSpecCa...rtNumber=60103

bluedawg

Ok so just letting the initial land were it does only works when it falls were it's needs to be to efficiently burn all the fuel. With 227*@.050" I'd figure you'd want 18 to 20 degreed advance timing at idle plus your vaccum advance on top of that. What rpm are you idling at. If you use the light springs it will get you a few extra degrees at idle. Lars papers would point you in the right direction.