I am not entirely sure the retard was just for emissions. I suspect it was also for a nice idle. It basically creates a second curve for the distributor. You can start the car nice and easy with no throttle input and as soon as you start to accelerate the timing jumps to a point where your idle would have been obnoxiously high at closed throttle. Advanced ignition with extra air at start isn't the best start scenario. It works but it isn't exactly refined.

But there is unburnt fuel on that graph. That’s what each of those gases are, combustion byproducts from burning hydrocarbons in air. I have no understanding of the chemistry, but I can see that stoichiometric combustion maximizes CO2 and minimizes other gases.

So, when you’re rich, there’s not enough air to burn all the added fuel. What results is incomplete combustion and a not so hot flame. When you’re lean, there’s not enough fuel to consume all the air. What results there is most likely detonation. That’s why O2 climbs in both Directions away from stoichiometric.

Fuel air mix is kinda like setting an oxy-acetylene torch. But with one massive difference, a moving engine. Compression and the act of compression heats the fuel air mixture, and the faster the piston moves, the more aggressively it’s heated. So, the spark can ignite the mixture sooner… further and further before TDC aka maximum compression.

The case I’m making here is that when you ignite the mix matters just as much as the mix you ignite.

Brian says he has an 11.7 AFR at idle, which is decently rich. But can you see how much more rich your AFR would read if you advanced the spark timing even further?

Now that I know what his AFR at idle, I think the AFM needs to be leaned out AND the spark advance curve corrected. Just some regular tuning… but I still think Brian should start with getting the aftermarket distributor to work like OE as much as possible for all round performance and drivability.

i think in the case of the L jets it was for emissions.
makes them run hotter than ideal at idle.
so its not the best point for idle.
and they are known for it.

you are correct with all the rest of it.
starts easily at the ideal point. no vac. no retard. then goes to retard.
and yes snaps straight off when you open the throttle so you get the ideal advance curve of the cent. dist operation.

its a kind of exaggeration of the scenario you describe.
they just go for more retard than is "ideal". or its "ideal" when it comes to NOX.
its NOX they were going after. main ingredient of photo chemical smog.
so yep, makes it easier to start.
snaps off for good response when you put the boot in.
but you are coming from even further back on the back foot.

digesting.

Let me clarify, my engine does not idle at 11.7 I was asking if the 11.7 on the chart correlated with afr on my gauge readings. My engine idles at 13.5 but idles better when richer. Any leaner makes for a rough idle.

The curve I programmed into the 123 was a copy of the proper factory curve with advance but no retard.

i know that chart co-relation i did was a bit rough.
because i just equated CO levels to what i had measured back in 90.
i was just fooling around.
but i was in about the same vicinity as you at idle?

if i used CO2 on the same chart (easy to get off my emission test back then - just subtract CO from the CO + CO2 percentages) it doesn't co-relate to the same AFR i get off the chart as the CO. its a little different. but it is still in that vicinity.
by 11.7 do you mean the 11.17 i quoted from my emission test of 1990for idle CO + CO2? in which case I can find both curves or lines. i have the CO - which was 1.88% (gee i was .12% under the factory manual, not good) and i had the CO +C02% as 11.17%, so i could derive a CO2 % if i wanted to of 11,17 - 1.88 = 9.29%. and then i could go on to Van's graph and look at 9.29% CO2 line and see where that landed me on AFR. which would be 12.0 and under. 11.7? thats richish. thats what i mean. but i'm not sure how you can go directly from those measures i had in 1990 to the graph that Van posted. thats for a certain type of fuel. i was just doing a ball park thing.

they did apparently set them up a little richer as air cooled engines.

geez, i don't know if i want to go into this O2 territory with mine and probes!
but i am counting on Van to make the mud settle to the bottom of the pond for me.
.....i am an old bastard. trying to keep up. waste of time?

i'll leave Van to comment on not incorporating retard.
but its going to have an effect.
i think what it means is once it comes off vacuum our mechanical advance is like yours.
but on the way back down its different in terms of timing.

and then we have vac advance. whole other thing?
but thats just at cruise? i believe. not when your foot is mashed. its all mechinical again when your exercising your right foot downwards.

Brian, good to know. To be clear, I don’t have high confidence on how great AFRs would be on a prolonged decel. My interest in this topic is because as a mediocre superbike racer many years ago, I learned the value of a good off-on throttle transition.

What we would tune for was a little richer on decel for added cylinder cooling, but not so much that it would load up/bog at the first touch of the throttle. You also didn’t want it so lean that temps were high and you would get that detonation pop as soon as you dumped more fuel and air in.

I’m getting off topic here, it what I’m driving at is, without knowing what other L-Jets work like or what the factory intended, I can only assume that the engine should run mildly rich on a closed throttle decel.

And, I’m not sure if retarding the timing on closed throttle will get you all the way there, but it will get you closer.

@wonkipop for your educational purposes question. Retarding the timing means the spark is delivered closer to TDC, but always before TDC. To paint two ends of the spectrum, if your engine is running 5000RPM at WOT, you are sucking in all the air possible, the fuel injectors are working at full steam to add a matching amount of fuel, and the pistons are smashing that mixture into a hot cylinder as aggressively as it can. In this scenario you need to advance the spark considerably so that you can ignite the mix at the right time to get a smooth even combustion burn.

Conversely, if you are closed throttle at 5000RPM, there is very little air coming in, and accordingly, much less fuel. So, there is less to compress, which means less heating of the fuel air mix from compression. But there is still a very high piston speed to account for. So, you ignite that closed throttle mix sooner than at idle but not as soon as you would a WOT mix of the same RPM.

ok, now i see what you are driving at. i was down there at idle. but now i am up with you at high speed back off, but a f r is down at idle level. got you.

and of course that is what our standard distributors do.
immediately offer that retarding of the pure mechanical position.
to an ideal spot for that.

so i was understanding you. i was just somehow stuck on your advancing at idle analogy. but i get it now what you were saying.

have you ever been able to determine what the vac retard amount is in the literature.
it would be maximum retard at higher revs back off too in mine. no decel valve.
different with the decel valve as its breaking the vacuum and reducing it.
assuming that is at idle that the vac level isn't enough to fully move the retard position and only goes a part of the way of its travel.

Nothing in the manual but you can measure it with a timing light at idle I bet.

the easiest way to know that it is for emissions in L jet is the tune up procedure emery. time it with the hoses off the distributor and plugged. get it sweet.
then plug them back on. and do a final tweak of idle speed.

kind of tells you that you then turn yourself over to faith in the vac system of the distributor doing a distortion of that best state of tune according to its own devices.
with the pure intent of altering the NOX and HC popping out the tail pipe.

i can get it slowly bit by bit as i go along now after studying this graph.
part of what VW were doing was idle tuning/timing with the hoses off somewhere near where the NOX line and the CO line cross. the HC is also nearby as a minimumish level.

Click to view attachment

as to the precise chemistry of what then happens if you "artificially" shift that best state of tune with a vacuum induced retard i need to know more than perhaps i care to.
Van is more eloquent.
the result is even lower NOX than you would get at the tune with the hoses off and HC still gets burned, but less in combustion moment and remainder in the upper reaches of exhaust system and exhaust port of head. which is where the engine temp goes up. in those areas. hence the engine running hot at idle characteristic. the EPA was fixated on NOX to begin with. along with hydrocarbons.

I am coming to this system from ford's CFI system and that one is 10 years later and far more computer (hah) intensive, as well as epa strangled. Ford actually used the timing to set the idle on my mustang. If you check your timing at idle on an 84 cfi 302 HO it will often read 20 or so degrees advanced and 14.7 on the afr. Since then I have been very interested in idle stability and quality and timing. I have found, even on the 914 djet, that it is expecting a very specific timing for idle to work well. On my 914 it is 9 degrees advance. I checked it yesterday and 9 degrees at idle and 33-34ish all in is where it is at (assuming my light is any good and my marks are ok). It was around 29 at 3200. When I move off of that the idle gets unpredictable, not pleasant, and sometimes stinky. When I had the original dizzy on there with advance and retard, it was a real difficult time getting everything to play right but I suspect that was mostly the bad triggers.

I've learned that adaptive dwell is like a cheat code for your distributor.
Seriously, I am so f'n pleased with the hall effect setup of my pertronix

Yeah, Ford guys complain but the tfi hall effect dizzy is quite an improvement.

Odd thing is that most mechanics screw up timing a cfi mustang because they don't unplug a little wire from the tfi module that tells the computer to advance the timing at idle. If you do it right on that car it is supposed to be 8 degrees adv with the wire unplugged. Most of us push it to 9 or 10. I completely forgot about that (my mustang hasn't needed my help in several years). Interesting that the base timing on those would be 8-10 but then the computer takes over, Jacks the timing way up, and probably does some magic with the afr and you end up at 850 rpm in neutral.

If anything, mucking with these early systems is really making me appreciate both the early engineering and the later engineering. My 84 gt350 is the last year before the dreaded "engine light".

i inspected our 80s vintage gizmo analyser, tuner with a screen etc the size of a doctor who dalek that sits in the corner.

the screen bit is kaput, needs some vintage expert to fix that bit up.
some kind of oscilliscipe sci fi thing.
but the exhaust gas analyzer bit still works.
does 4 types of gases and ingredients.

CO

CO2

HC

i think it does NOx, i think that is what mike said.

has no scope for oxygen because thats something you breathe?
and bosch were holding the patent for those probes close to their chest even in the 80s.

i'm going to fire it up on the next tune up.
going old school. i'm an old man so its legit.
be catching up on technology just using the machine?
i used to static time my square back with a light bulb. (quick duck a peradactyle is swooping us).
onward into the space age after its already gone.

Finally got my new O2 sensor. On decel mine is running between 12 and 13. More towards 13 most of the time. Looks like it's kind of binary so I think something specific is occurring (mps/decel combo) and it is steady across the decel. So definitely not full rich.

you're definitely in a good spot.
I know we're talking about D-jet vs L-jet here, but you and Brian have the same distributor. Do you know what your advance is in a closed throttle/decel situation?

It would be entirely dependent on the motor rpm and the curve built into the 123, the vacuum advance is hooked up correctly so on decel there will be no vacuum there. I have the potted version so I can't check my advance through blue tooth.