Tuning Webers using an AFM - experiences?

[lkadams]

With my engine out over the winter, I welded a bung into my exhaust to run a Bosch Wideband O2 sensor on an AEM AFM to monitor AFR (that's a lot of TLAs in a row).

So my question is - now what? LOL. I've been able to validate that I'm running a touch on the rich side (to be safe) and that under light throttle at about 3000 rpm I get some lean backfires on decel (which I find funny that I'm rich everywhere, except there - is that emulsion tubes?). So it confirms what my ears are telling me. Is there a good resource to correct AFR readings at throttle behaviours vis-a-vis what's symptoms with idle jets vs emulsion tubes vs. air correctors vs. main jets kind of thing.

In other words, I'd like to use my AFM to narrow in on what small adjustments I need to make to optimize my setup. What AFR is ideal at light throttle, cruising, etc.

As I have to say at work - sorry, I'm an engineer, and I'm here to "help"....

[Richy]

What is your engine ? what carburetors do you have ? Can't answer the question without this info , and maybe still can't .

[uai]

I'd say if you're not below 11.8 and above 14.7 everything's fine.
Apart from that it gets scientific. It's best to do it on a dyno and ignition should be taken into consideration as well.
However I did all the jetting on the road before going on the dyno and there just went to look with ±1° and ±2° ignition and ±5 mains and ± 10 air corrector jets for the most power as every engine is a little different.
Doing it on the road is dangerous if you do not have all the equipment needed which means Data aquisition/Logging for RPM, Throttle Position and A/F Ratio. Especially TP helps in tuning because then you can look where you're in transition.
There have been written whole books about the tuning of webers
(you can even an find some scans online search term could be e.g "Power Tuning Webers V2 (clear images).pdf" or "Weber Carburettors A Guide to Tuning 1976.pdf" or "WeberTuning Manual 1985.pdf")
Also depending on your starting point you may end up spending a lot more on jets, venturis, and all that stuff than a professional with a dyno billing only for the jets he uses in the end.
Max performance you'll have between 12.2 und 12.5 lean stumble/hesistation may appear from 14.X onwards
You should take into consideration that you do not want to be lean when running full throttle high rpm as fuel also cools the engine.
But the most important thing do not crash while staring on your AFM when driving on the road.
Do you have 2 lambda sensors? one for the left and one for the right bank?
https://www.yumpu.com/en/document/vi...wer-by-bmw-e21

[dmaddox]

My advice is to ensure good ignition, ensure good fuel filter/pump - and then tune the webers by the books, ignore the AFR until you get the carbs dialed in....THEN look at your AFR for readings to determine fuel mixture along the rpm band. Tuning solely based on AFR readings is going to drive you crazy.

[lkadams]

I appreciate the insights. To be clear, I am not talking about doing a baseline setup on Webers using an AFM, I am wondering about fine-tuning once they have been set up following all the conventional wisdom (lean best idle, etc.).

So for those who had a properly setup set of Webers (to build on what Uai is saying), and have checked the AFR, what do you see for:
- AFR at idle (900 rpm)
- AFR under "cruising loads"
- AFR under acceleration

Uai you had a great point about timing as well - I installed a 123Ignition Bluetooth distributor as well (the most high tech thing on the whole 52 year old car!) so timing adjustments are in the mix as well.

[lkadams]

Just as a reference for anyone else who may stumble into this, there's an excellent thread on the bird with some studies in tuning of PMOs using AFM
http://forums.pelicanparts.com/911-e...rb-tuning.html

[edmayo]

And bear in mind that exhaust leaks can cause deceleration backfires