What are ideal A/F ratios for different driving conditions?

[JFENG]

As some of you know I have an old Jag (I call the condition pre-restoration).
I installed a spare/new motor plus a wideband AF sensor, and am tuning the carbs by adjusting the needle profiles (SU's use a tapered needle where length along the needle correspond to intake airflow rates). Simple to tune because you only change one thing. But not possible to fully optimize like a DCOE.

Without modification but with new needles/jets here is where I am.
IDLE: 14.5 to 1

Light throttle cruise: 13.4 to 13.7 depending on rpm. Above 3500rpm it drops to around 13:1.

Medium throttle: 13.3 to about 13.5 to 1

Wide open throttle: 13:1 below 2000rpm gradually richening up to 12.4 to 1 above 4000rpm redline is 5500rpm).

Previously with the original 60 year old needles it was pretty much the opposite of this (14.5 at WOT and 12.5 to 1 at idle). Plugs would foul up around town, PITA.

What would you guys focus on changing first and what target AF ratios would you go for?
Should I go much leaner at idle?
Is 13.5 too rich when I'm cruising down the highway at 85mph?
John

 

[Stevehose]

I wouldn't touch a thing if the engine is happy, those numbers are great for a carbureted car that old. I am surprised it likes to idle at 14.5, usually it's richer but that's great.

 

[x_atlas0]

I can tell you for modern engines, the target is to be at stoich unless you are in a high-load zone. (high RPM, high pedal demand, or both) Over stoich you are just wasting the fuel unless you need it for anti-knock (which is usually more tied to timing on a NA engine) or for peak power. (peak power AFR is ~12:1 NA) Of course, this is on a modern FI engine, so the older ones may appreciate some extra fuel, and it may give some additional responsiveness.

 

[JFENG]

I wouldn't touch a thing if the engine is happy, those numbers are great for a carbureted car that old. I am surprised it likes to idle at 14.5, usually it's richer but that's great.

Best idle is indeed richer. But who spends that much time on the throttle stops. What I want is smoothness around town and not to burn a valve when I have to keep it above 3500rpm trying to keep up with more modern classics.

You know the whole reason I'm screwing with it is because you seem to have gotten your carbs really well setup, and you showed us the details matter.

 

[Stevehose]

From what I've researched on old carbed cars, idle AFR is not that important, as long as it is within 12-13-14 and the engine is smooth, set it where the engine likes it. There is exhaust gas dillution at idle so the reading may not be correct anyway. I can put in 5 different sized idle jets and my engine will always want to idle at 12. More important is the off-idle transition, WOT and cruise. You're in perfect zone for WOT and cruise at 13.5 highway is good - you won't burn a piston/valve or wash your cylinders with gas and your mpg will be ok.

Carbs are all about comprimise, if you start shooting for stoich cruise then your transition or WOT will be affected, those numbers you have are very good. Just set your idle to where the engine is smooth and happy, check that it doesn't affect your transition too much either way (sometimes the idle mix screw is used to weaken or strengthen mixture on progression as a band aid). Then you're good to go.

 

[Sven]

Yes, I agree, those numbers look great. You are set. At WOT you want to max out at 12.5 (which gives the most power) Any richer and you start washing cylinder walls with the extra fuel. Ideally you could be bit leaner at highway cruise (light load), but that is a small compromise.

As an aside, I am not sure why a carb engine would need a higher AFR than a FI engine for a given load. As far as the combustion chamber/pistons are concerned it doesn't know how it got the fuel.

 

[61porsche]

SU's

can be tuned. The factory did.

The needle needs to be marked in 1/8" increments. Likewise the piston needs to be measured. Both positions are relative to vacuum and rpm. Yes, vacuum, so you need a gauge and a way to observe the location of the piston. Dyno, go cam, etc.

Once you can determine a given range for a given situation you can sand( remove) and alter the needle or start with a fatter needle to lean that rpm. Micrometers are necessary.

The accelerator pump in a zenith is the dashpot oil, spring, and damper on an SU. Those were set up for 20 weight oil. Not ATF, 3 in 1, or anything else. There normally are or were 8 or more different dampening springs to choose from.

 

[jamesw]

I think the A/F numbers you are at for your various throttle positions and load are pretty darn good. I would leave it like that personally.

I've tuned many an SU on my old Datsun Roadsters...

Cheers
James

 

[JFENG]

Doesn't see a bit rich to you guysfor light load conditions? I'm thinking I can lean out the idle to 15:1 or more as long as the idle is stable. This will get the light load running closer to 14.7:1

 

[Stevehose]

Old engines + carburetors + bad modern gas = err on the safe side. You can chase the stoich dragon forever if you like the challenge, but I'd give up amy miniscule mpg for keeping the engine in tact and having full performance on tap. Stoich and higher will also make for higher head temperatures.

No reason to have the idle that lean, and the mixture screws shouldn't be used to lean out cruise afr (at least on dcoe's), if that is the case then your idle jets are too rich. But yours aren't and I would love those numbers. IMHO.

Doesn't see a bit rich to you guysfor light load conditions? I'm thinking I can lean out the idle to 15:1 or more as long as the idle is stable. This will get the light load running closer to 14.7:1