Lean Idle solutions?

[pauls428]

Have a 69 Pontiac 428 edelbrock rpm heads, intake, long tubes, stock cam, late 70s q jet, innovative motorsports wideband o2 sensor

been trying to tune, set float height and followed directions throughout rebuild using 76 jets and smallest primary rods silver power piston spring, anyway it was running lean at idle around 15 to 1, have the idle adjustment screws literally one turn from falling out, my solution has been raising  the seated position of the primary rods with the power piston stop, the car is drivable now but just barely at cruise still sits around that high 14 or 16 to 1 (too lean) wondering if the  idle solution is going to create a domino effect on the rest of tuning.

appreciate the input!

[Hillbillyenginering]

Just a FYI.
Cliff is going to want more info than this.
Ideal restrictions, idle bypass, etc,etrc. Carb number. The more data you give now, the less questions he will ask.
Also type in your carb number and research answers already given on here. The cubic inches. There is a lot of info on here if you researecg it.
Sounds likje you haven't opened up your idle or by pass circuits enough.

[carmantx]

It does sound like you need to richen the idle circuits, but as mentioned, lots more specific info needed.  "Smallest" primary rod isnt a good description.

[Cliff Ruggles]

Compression ratio, cam specs, carb number and exactly what's been done to it required.

Most likely the later model emission calibrated carburetor simply doesn't have nearly enough idle fuel to the mixture screws for what you are doing.

Each part number is application specific, so specific guidance and tuning methods are required.  Might want to start by getting one of our books and becoming familiar with q-jets, it will save a lot of typing and confusion here if/when I would ask specific questions about the carb you are working with, etc......Cliff

[pauls428]

lets get caught up....
here's some specs.. carb... 17059253, compression 10.3 to 1 (according to math including thicker head gaskets) primary rods .044 x .026, 76 main jet, da secondary rods with h hanger(NOT ADJUSTED) idle as of now is 650 rpm.
cam=either speed pro 1022 or 1038 timing at idle is 3 degrees atdc no vacuum advance
then an email (FROM CLIFF) a long time ago in a galaxy far away
Things just don't work that way with engines.  Part of a successful engine is careful and correct cam choice.

When the cam is too small it will lack cylinder filling abilities, and make to much cylinder pressure early in the rpm range (detonation).

When the cam is too big, idle quality suffers, loss of throttle response, off idle power, etc.

You cam will work fine, idle should be fantastic, but it will require very close control of timing/fuel curves to avoid pinging at that compression ratio, even with aluminum heads.

Good news is that I can provide the rebuild and tuning parts to get the carb up to par for what you are doing.  The careful tuning will have to come on your end, and hopefully you can make the cam work well enough to manage pump fuel and enjoy the engine on the street.(WORKING ON IT

[carmantx]

What size are your idle tubes and down channel restrictions?  That is a 79 403 Quadrajet.  Probably lean on idle circuits and small holes under air idle screws.

[Cliff Ruggles]

76 jets are HUGE in one of those units. 

For certain increase the idle tube size and open up the idle down channel restrictions to give the engine more idle fuel.

We don't sell a "silver" PP spring, so can't advise there. 

The 79 carb will have fine pitch metric threads for the idle mixture screws, so not uncommon to have to back them out 4-5 turns or so to start finding the idle fuel you need, compared to 2-3.5 turns out for the earlier units with 10-32 threaded screws......Cliff

[77cruiser]

Gotta ask about your timing though. Looks like you need to add about 20 degrees.

[pauls428]

they say most carb problems are timing problems, like I said at idle 3* timing not using vacuum advance if I remember correctly (and don't hate me for this) but the cam was installed a degree and a half retarded. ill have to borrow the timing light from work and find a spec to work from..... as far as idle tube restrictions I am not really set up or all that comfortable to modify that, I will measure them and get back about it.... as much as a pita this has been it has been interesting learning how the circuits overlap and can compensate for one another.... how the car runs differently. on the other hand, maybe just another q jet is in order.

[RLF]

I could never get mine to idle any richer than 14.5:1 on the AFR meter but I noticed the exhaust was getting blacker as I richened up the idle circuit. For whatever reason (maybe large cam) it always reads lean at idle even though its rich.
If it idles strong I wouldn't be too concerned, same with your lean cruise as long as the AFR's richen up with power piston movement.
You do have ignition timing to sort out though!