General tuning questions and idle AFR

[travis]

I’m probably going to regret ever buying an a/f gauge. While it seems to be great for tuning some things that are way out of whack, for other things not so much. I may be doing it wrong...wouldn’t be the first time.

Tuning the 650 edelbrock on my 351w truck engine, getting the idle afr anywhere close to mid 14’s makes the idle raggedy and sounds like crap. I quit trying to hit any certain number and just tuned it the way I’ve always done...with a vacuum gauge and a tach. After getting it tuned for best vacuum at a 700-750 rpm idle (18-18.5” vacuum), it idles quite smoothly (as I would expect it to with the cam I’m using), but the idle afr is in the low-mid 12’s. I haven’t pulled any plugs out yet, but it doesn’t smell rich at all, starts super easy hot or cold and will idle all day long if desired. It also will go from dead cold to a stable idle by itself (no choke) much more quickly than before.

Is this fat idle afr telling me anything? Pointing out something wrong with my tune? Leaning out the idle mix screws to low-mid 14’s drops idle vacuum by 1-2” and diminishes the idle quality.

Cruise afr is 14.7-15.1, WOT 12.5-12.7. It drives better than ever now, so I think I’m getting close.

I’m going to try some more tweaks on the timing and timing curve. Would you expect timing changes to change the afr’s significantly?

[Belgian1979]

Generally said an AFR gauge can lie to you. If you have any leaks in the system it will show up as a lean condition. So you'll need to drive up the AFR in order to compensate despite the engine maybe still running close to 14 - mid 14/1.

Beware that the current fuel usually has some ethanol in it, which means the stoich ratio is closer to 14.2/1.
With a large cam things are compounded and you generally need a richer mixture. This is due to the increased overlap. A lot of unburnt mixture goes out of the exhaust showing up as lean because of the O2 content.

[steve cowan]

I shoot for 8-10% richer than stoich for pump gas, at WOT
My opinion is if engine cold starts and idles its to rich.
Placement of sensor makes a difference and positioning of sensor in relation to angle of sensor as well

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[F-BIRD'88]

Carbed engines DO NOT IDLE AT 14.7:1 ...

Carbed engines idle at a afr that is RICHER THAN 14.7:1.

At idle it wants what it wants to idle best.
12.5:1 +/- is normal.. Ignor the AFR guage at idle,

If you did have a heated plenum and ribs/crosshatch on the plenum floor it may (or may not) idle a bit leaner, but again ignore the AFR guage at idle. (It would have better idle quality and also generally drive BETTER as more fuel gets vapourized.)

[travis]

The cam in this is a small hydraulic roller (from Camking)...264/268@.006" on a 112 lsa, 218/222@.050. So its not big at all.

The O2 sensor is in the collector on my headers, angled properly. I don't believe I have any exhaust leaks

FBird, changing intakes at some point will probably happen, but it won't be anytime real soon. Nothing is available at the moment other than some horrifically overpriced bling intakes, or some Professional Products stuff, or some that are way, way wrong for a truck.

It won't start and idle on its own right away...it needs 20-30 seconds of dancing on the throttle before it will idle on its own. At least that's the way it was yesterday with some very unseasonably cool weather (low 50's).

[F-BIRD'88]

Thats normal cold start up.. A choke and fast idle linkage helps, Do not allow it to idle and struggle at low rpm when cold during cold start up . Raise the idle up 1200 -1500 rpm when cold so it does not struggle (that fouls the plugs)

Thats what the choke and fast idle system is for... Hook it up and use it.. If manual choke get a choke cable kit.

Such a cam can use up to 24 base initial timing.
(more is better) It wants what it wants..
Especially if non heated intake manifold plenum..

A custom heated type carb base spacer (with temp control) is an alternative Warms the carb up. Aids fuel vapourization

[F-BIRD'88]

Again do not allow it to idle cold by itself at low rpm.
Keep your foot on it and rpm up for a few minutes or hook up the choke and use it.. All that OEM type stuff has a critical valid purpose.

[novadude]

Ignore the gauge at idle. Richer idle mixtures are normal and necessary. You have less density with a closed throttle, a lot of exhaust residual from high intake vacuum during overlap, etc. You need a richer mixture for smooth operation.

A text book, "internal combustions engines and air pollution" by Obert has an excellent A/F vs load graph that shows what is roughly a "U" shape as desirable for best operation (A/F on y-axis, load on X-axis). I don't have a copy of that image right now, but someone may be able to share. Rich at idle and WOT, and lean at all part throttle points in between.

[F-BIRD'88]

You may also find significant benefit by re installing the Ford
OEM origional air cleaner wiyh the vac/temp controlled heat/cool air diverter valve in the inlet snorkle.
You can make up a new sheet metal "heat stove" on one of the exhaust header tubes to duct hot air up to the temp controlled air cleaner. Warm air at idle and cruise (especially when cold) makes a big difference.
.
The vacuum valve opens at WOT .

[77cruiser]

If you try to lean it more you will get the stinky eye burning exhaust that most associate with being rich.

[dannobee]

F-Bird is right. Chokes and pull-offs are there for a reason, make sure all of that stuff is adjusted correctly.

The O2 sensor is likely not staying hot enough at idle if it's in the collector.

But more basic, try adjusting the idle mixture using the "lean drop" method instead of the AFR gauge.

[Tuner]

I’m probably going to regret ever buying an a/f gauge. While it seems to be great for tuning some things that are way out of whack, for other things not so much. I may be doing it wrong...wouldn’t be the first time.

Tuning the 650 edelbrock on my 351w truck engine, getting the idle afr anywhere close to mid 14’s makes the idle raggedy and sounds like crap. I quit trying to hit any certain number and just tuned it the way I’ve always done...with a vacuum gauge and a tach. After getting it tuned for best vacuum at a 700-750 rpm idle (18-18.5” vacuum), it idles quite smoothly (as I would expect it to with the cam I’m using), but the idle afr is in the low-mid 12’s. I haven’t pulled any plugs out yet, but it doesn’t smell rich at all, starts super easy hot or cold and will idle all day long if desired. It also will go from dead cold to a stable idle by itself (no choke) much more quickly than before.

Is this fat idle afr telling me anything? Pointing out something wrong with my tune? Leaning out the idle mix screws to low-mid 14’s drops idle vacuum by 1-2” and diminishes the idle quality.

Cruise afr is 14.7-15.1, WOT 12.5-12.7. It drives better than ever now, so I think I’m getting close.

You are on the right path, continue to follow your intuition.

I’m going to try some more tweaks on the timing and timing curve. Would you expect timing changes to change the afr’s significantly?

HaHa Yes. Tune the symptoms first and reference the gauge to keep track that the changes have logical effects. If the numbers on the gauge get out of a "normal" range toward lean look for some other thing to be affecting the number, like a vacuum leak, exhaust leak or bad plug wire.