[Shiny]

Sorry if this is basic to most of you, but here's my random question for the day following recent attempts to learn about and getting help with both my distributor and carb tuning.

If the A/F is being measured AFTER combustion, does the timing affect the readings or is it only affected by the A/F mixture?

Does an A/F meter measure CO or CO2 to O2 ratios (combustion products) or can it also sense unburned gas (total hydrocarbons)?

Given the "degree of combustion" is controlled by both timing, gas/air ratios, and loads on the engine, how do you use an A/F meter to separate timing from carb tuning?

My impression is A/F meters are used to optimize carb and/or FI tuning after a timing curve is set, which suggests timing and fuel/air ratios are somewhat independent ????

[AG]

First set your timing for best mph (if racing) then tune for your desired AFR during idle/transition and WOT.

[Shiny]

Thanks AG

It's just now barely running and will never race, but I screwed up my distributor and battled an unproven frankencarb until forum members and shaker455 brought me to a better place.

I have read the same advice about setting timing first, then the carb tuning...

But the vacuum advance education has me wondering about how the timing affects A/F ratios.

[Kenth]

For better understanding of the combustion engines different functions, i recommend getting Doug Roe´s book "Rochester Carburetors" and download the pdf in this link:

https://radionerds.com/index.php/Fil...E_VEHICLES.pdf

Spend some time with this literature, it´s correct, good reading and will save me from writing a book here.

HTH

[JLMounce]

This simple answer is that yes, timing will effect AFR because it can effect the amount of burn the combustion process produces.

For a carbureted vehicle, I would absolutely set timing first based on WOT operation and what is needed at idle + the curve. Then tune the carburetor for idle and WOT. Your AFR at cruise and idle will be somewhat less important because this is low load situation. further idle and cruise tuning can produce more economy if you're worried about it.

[johnta1]

No, AFR may influence how the timing affects combustion, but the AFR is already set when ignition occurs.
A certain AFR may need different timing requirements to make full combustion, but unburned fuel is not a 'rich' or 'lean' AFR.
It's incomplete combustion.

Different fuel properties like % of alcohol in gasoline or other qualities will require different timing and it probably will require different AFR.
(like altitude, temperature, etc will cause different AFR requirements possibly)

[Shiny]

Originally Posted by Kenth

For better understanding of the combustion engines different functions, i recommend getting Doug Roe´s book "Rochester Carburetors" and download the pdf in this link:

https://radionerds.com/index.php/Fil...E_VEHICLES.pdf

Spend some time with this literature, it´s correct, good reading and will save me from writing a book here.

HTH

Thanks Kenth

I'll chase it down as it sounds like a good book.

I've gained enough understanding to ask the question I asked, which was really about how an A/F meter in the exhaust could distinguish between the effects of a/f mixture in the carb and the effect of timing on the combustion of that mixture.

Hope this makes sense.

Originally Posted by JLMounce

This simple answer is that yes, timing will effect AFR because it can effect the amount of burn the combustion process produces.

This is where I'm poking. I understand both timing and AFR will affect the degree of combustion but not sure if those two can be independently sensed downstream in the exhaust.

For a carbureted vehicle, I would absolutely set timing first based on WOT operation and what is needed at idle + the curve. Then tune the carburetor for idle and WOT. Your AFR at cruise and idle will be somewhat less important because this is low load situation. further idle and cruise tuning can produce more economy if you're worried about it.

This is consistent with what I've been ingesting and seems to be the practical approach.. thanks!

Originally Posted by johnta1

No, AFR may influence how the timing affects combustion, but the AFR is already set when ignition occurs.
A certain AFR may need different timing requirements to make full combustion, but unburned fuel is not a 'rich' or 'lean' AFR.
It's incomplete combustion.

Different fuel properties like % of alcohol in gasoline or other qualities will require different timing and it probably will require different AFR.
(like altitude, temperature, etc will cause different AFR requirements possibly)

This is again at "the heart" of my question. AFR sounds like something controlled upstream of the combustion, yet AFR meters seem to measure stuff after combustion. Does this mean the "typical" A/F meter in the exhaust stream can tell if there is incomplete combustion? Are they measuring combustion products (ie CO, CO2) or do they measure "total" carbon that might include unburned hydrocarbons?

Mike

[25stevem]

The first thing that needs to be confirmed is that the spark plug is in the ball park for not being too cold and as full throttle is approached that it’s not too hot.

[johnta1]

Most EFI should have systems before and after the engine.
Like the Mass Flow sensor which reads the air (O2) and the O2 sensor at the exhaust. (which should be close to the exhaust port for accurate measurement)

With calculations it determines what is combusted. (and a lot more data input probably)

The AFR meter's usually just read the O2 in the stream. If you use alcohol for fuel it would be way off unless it can read Lambda. (which is converted to AFR for the fuel )
They also probably are based on complete combustion and correct timing etc.

If anything is off it is basically GIGO?

[JLMounce]

Originally Posted by Shiny

This is again at "the heart" of my question. AFR sounds like something controlled upstream of the combustion, yet AFR meters seem to measure stuff after combustion. Does this mean the "typical" A/F meter in the exhaust stream can tell if there is incomplete combustion? Are they measuring combustion products (ie CO, CO2) or do they measure "total" carbon that might include unburned hydrocarbons?

Mike

This is one of the pitfalls of tuning with an AFR sensor, specific to carbureted vehicles. Johnta1 is correct in that the AFR an ingestions is going to be based on whatever is going on with the carburetor. Nothing the spark plug or when it fires is necessarily going to change that.

However, the completeness of the combustion will change the reading on the AFR gauge, which is why you'll see some variation as variable factors shape what is happening in the chamber.

All this of course assumes that there's no exhaust leaks near the oxygen sensor that can skew the metered results from actual conditions. For this reason, specifically with a carbureted engine, reading plugs is still essential.

With an EFI system, you can change the amount of fuel on the fly which is why downstream reading becomes more important.

As far as what the oxygen sensor measures, it actually takes a variance of the amount of oxygen inside the pipe and compares it to the oxygen outside of the pipe. This difference creates oxygen ions that create voltage. The sensor will work in lambda where 1 = 14.7:1 (stoich). readings below lambda are rich, readings above lambda are lean.

[Shiny]

Great replies, thank you all for taking the time to explain.

[Ram Air IV Jack]

Cliff Ruggles is the Qjet expert and posts quite a bit here. Cliff wrote several books on the Qjet and knows them inside and out plus runs a parts business now for many carburetors. Cliff is the guy to consult on carburetor issues!!!

https://cliffshighperformance.com/

[Schurkey]

Originally Posted by JLMounce

As far as what the oxygen sensor measures, it actually takes a variance of the amount of oxygen inside the pipe and compares it to the oxygen outside of the pipe. This difference creates oxygen ions that create voltage. The sensor will work in lambda where 1 = 14.7:1 (stoich). readings below lambda are rich, readings above lambda are lean.

There's exhaust-gas analyzers (shop equipment) that can measure multiple components of exhaust gas. A "5-gas" unit would measure hydrocarbon content, CO, CO2, Oxygen, Oxides of Nitrogen (NOx). A "4-gas" machine probably leaves out NOx.

I worked in a shop that had an ancient machine, measured CO only. Very useful for rich/lean tuning of idle mixture screws.

The modern hand-held or in-vehicle units use a wide-band O2 sensor. As said, it measures exhaust oxygen in comparison to oxygen outside of the exhaust pipe, which believe it or not, can be delivered to the sensor via the gap between the wire strands inside the insulation in the wire harness of the sensor.

Rich/lean (AFR) is calculated from O2 content, rich/lean is not measured directly.

Because an O2 sensor does NOT measure hydrocarbon, or CO, or CO2, or NOx, misfire will cause a false "lean" reading. The oxygen that should have combined with the hydro-carbon fuel to produce water vapor (H2O, hydrogen + oxygen) and CO2, (carbon + oxygen) doesn't, leading to high exhaust O2 content.

Similarly, any vehicle with an O2 sensor cannot have Air Injection Reaction equipment pumping air upstream of the O2 sensor, or even downstream but close to the sensor. Again, you'd have a false "lean" indication from the sensor. Those vehicles that do use an AIR pump, supply air ahead of the O2 sensor only during "open loop" operation, when the computer is ignoring the O2 sensor anyway.

[Shiny]

Shurkey,

Thank you, I think that adds another layer to the other posts.

I remember state vehicle inspection stations could measure hydrocarbons at the tailpipe, so was probably similar to your 4- or 5-gas systems.

Sounds like most of these monitors are reading only oxygen and I can assume they will have SOME sensitivity to timing, especially if the advance is too short to burn all the fuel.

But I can also guess these systems are much more sensitive and/or practically more useful for carb tuning than for setting a timing curve.

Maybe I'll try to find some "user guides" or "owners manuals" for some of these AFR monitoring systems. What are some popular "in the car" brands?

Mike

[Cliff R]

Tuning with a meter for AFR at idle is pretty much useless.

In any and all cases we need to give the engine the timing and fuel it needs to make it happy.

I'm not sure how folks are recommending timing for best WOT power or track performance first as total timing from the mechanicl curve in the distributor simply adds that number to initial timing.

I do prefer to set total timing at full power first, then see where the initial timing is at and modify the advance curve if/as needed so initial timing is right where it needs and I still have the same total timing that the engine wanted.

I've found that most of these engines (well thought builds) like about 10-14 degrees initial timing. Anytime you find yourself needing to run more initial timing the cam has more overlap or tighter LSA than is ideal for the CID/compression ratio. Nothing wrong with all that as most folks in and associated with this hobby love the "menacing" idle quality that developes from that scenario. Even so when you choose a cam that doesn't make ideal vacuum at idle you are just NOT going to find it easy to tune the carb and distributor and typically going to have to run a lot of timing at idle plus generous idle fuel delivery.

I've worked full time at helping folks out with that scenario now for most of my adult life. So back to AFR at idle speed. What we need to be concerned about more than AFR is idle quality and vacuum production. We need to find the ideal initial timing and idle fuel delivery so the engine starts and idles as good as it can based on the parts we've selected for it. That's pretty easy if we have optimum compression and minimal overlap as vacuum production will be strong and the engine woln't like, want, need or respond to a lot of timing at idle speed nor will it need massive idle fuel delivery or much of or any idle bypass air.

ALL of those things, needing lots of timing, idle fuel and bypass air or how we correct less than ideal vacuum production at low RPM's.

So basically we need to find the ideal settings, whatever they may be and the resulting AFR really isn't on the list. When you are finished tuning nothing at all wrong with looking at the AFR to see where you are at, but if you engine starts well, idles fine in and out of gear, doesn't "load up" with long periods at idle speed, and smooth right off idle you are done tuning in that area.

Folks also need to realize that initial timing is a HUGE player in vacuum production, especially with larger camshafts and tighter LSA. For example, if you are working with a 400 engine with 10 to 1 compression and 067 camshaft it's going to make PLENTY of vacuum without much timing in it. I've tuned scores of those early engines for pump fuel without much initial timing or having to add any timing at idle from the VA. Take the same exact engine and put a RAIV cam in it (popular choice for some reason I'll never understand) and you'll be lucky to make enough vacuum at idle to even keep it running without going clear off the scale with it. Folks that go that direction, or even worse put an even larger cam in it with much tighter LSA very quickly find themselves unable to effectively tune that engine at idle speed with anywhere close to stock timing and idle fuel delivery.

Anyhow, in any and all cases I do agree with setting total timing first at WOT so you don't detonate the engine, but you may find out that the resulting initial timing isn't ideal for your combination and you'll have to make some adjustments there as well.

Here's how that works. Let's say that your new engine build made best HP on the dyno and fastest runs at the track with 34 degrees total timing but the mechanical curve in the distributor is 26 degrees and at 8 degrees intial timing it isnt idling well or making much vacuum. You also find little if any sensitivity with the mixture screws and unable to get smooth idle, or stable idle in and out of gear. So you advance the intitial timing from 8 to 14 degrees and it's a LOT happier now. Vacuum jumped from hovering around 8-10" to much steadier at 13". So we now have a simple problem to correct.

The engine wants 34 total timing and loves 14 initial. We are at 8 and 34 with the current mechanical advance curve. The cure is simply to modify the mechanical curve for 10 degrees of advance (18 at the crank) instead of 13 degrees (26 at the crank).

Another option that is available (popular but I dont' do this often here) is to leave the mechanical advance curve right where it is at 13 degrees (26 at the crank) so we still have 8 initial and 34 total and add in the timing the engine wants by using manifold vacuum to the distributor Vacuum Advance. The problem with doing that is that you MUST obtain a VA that is "all-in" at a pretty "low" vacuum reading. Or our example above it would need to be less than 9" of vacuum. Most VA units start around 8-9" and "all-in" around 12-15" so you will find yourself buying a custom piece or "adjustable" VA to get this to work. The other problem with using MVA is that we will also have to "limit" how much it adds. These "custom" adjustable VA units usually do one or the other and not both so we will have to install a "limiter" of some sort to set how much VA timing is added.

For the most part I consider that a "crutch" fix for most engine combinations, but done correctly it will work just fine.

I'll add here that there is a "crowd" out there that will tell you that you absolutely have to do it that way. That simply comes from doing to much Internet research following poor advice from folks who really don't know what the problem is in the first place or how to correct it.

Then you have the third crowd out there who will tell you get rod of the VA all together, run more initial timing and install some sort of "kit" or super light distributor springs to get the timing "all-in" right off idle. This seem simple but keep in mind that you are still MISSING a lot of timing at cruise or in the "normal" driving range so engine efficiency and fuel economy will not be at their best.

In any case I've been tuning these engine now coming up on half a Century and many thousands of them, and have NEVER once had any success at all with that deal, and 80 percent of the time don't need to use MVA to cure idle issues either.

So my advice far as this topic is concerned is to but a piece of black electrical tape over your gauge. Then find the best settings for your engine in terms of total timing, initial timing and how much VA timing the engine wants. Then remove the tape, observe your AF readings and make small changes from there as see if you can make any improvements anyplace. I'll add here that setting the idle mixture is the easiest part because it involves nothing more that setting curb idle speed and turning the mixture screws for the best idle quality at the leanest settings..........hope this helps some........

[Formulajones]

Originally Posted by Shiny

Shurkey,



Sounds like most of these monitors are reading only oxygen and I can assume they will have SOME sensitivity to timing, especially if the advance is too short to burn all the fuel.

But I can also guess these systems are much more sensitive and/or practically more useful for carb tuning than for setting a timing curve.

Maybe I'll try to find some "user guides" or "owners manuals" for some of these AFR monitoring systems. What are some popular "in the car" brands?

Mike

Yes, the AFR readings will change to some degree when playing around with timing.

Basically the way I've approached tuning for decades is to get the timing curve set first. Find total where the engine makes best power just as Cliff explained, and then work on the other half of it, getting the initial and the curve itself dialed in that makes the engine most happy. I don't pay much attention to AFR at all during this phase.

Once that is set, only then will I dive into the carb tuning aspect of things. Generally finding where the engine responds and drives the best with very little influence on the AFR gauge to start with. Once drivability is where it needs to be I'll tweak the AFR readings from there to see if I can find the "edge" and use that as a reference. Every engine is completely different.

Where people make the biggest mistake, and I still see this all the time, is trying to dial in a carb and going round and round with it, complaining about drivability issues before they have the timing curve where it needs to be, and then blame the carb for the problems they are having. Once I go back and go through the timing events first, it always cleans up the carb issues they were having. Never try to dial a carb in first until you are certain the timing is where it needs to be and functioning correctly for the particular combo you're working on. It'll save all kinds of headaches later.

[ta man]

Great reply Cliff.

[Shiny]

Thanks to all for the great responses.

My question was just curiosity as I don't have, nor am I likely to have a way to measure AFR. I had been reading about vacuum advance and the combined effect of AFR and engine loads on timing. So I was just wondering if the AFR monitors could separate the two.

But all of the tuning suggestions are definitely helpful and honestly, would be great to combine in a "Tuning" sticky. For example, Cliff recently layed out an approach to setting base timing in a different thread.

I am currently trying to extract myself from a self-inflicted cluster and am always grateful to you for helping.

Mike

[Cliff R]

Tks.

Couple of minor things to add.

Close to 8 out of 10 "carburetor problems" are part or fully caused by "ignition/timing problems".

When we "modify" carburetors, especially the idle system we are NOT making things richer far as A/F is concerned. 99 times out of 100 the choices made in the engine build REDUCE signal to the carburetor at idle speed. This does two things, It lowers engine vacuum AND the carb is not longer to able enough fuel to the engine via the idle system to make it happy.

We open up IFR's, DCR's, smaller IAB's, more bypass air etc simply so the carbs idle system can now supply enough fuel at the idle mixture screws based on the much lower signal produced at idle speed by the engine.

We do this in conjunction with MORE initial timing which helps burn a leaner mixture and it increases vacuum at the same time.

Doing one without the other very quickly gets the tuner into trouble.

For example, if your engine will NOT idle fully on the idle system, very little if any control with the mixture screws, etc, and opening up th e idle system helps you are most likely providing too much fuel to the engine at this point. What you should have done at the same time was to add initial timing to help the scenario AND went into the carb if/as needed.

Another thing folks get all caught up on is they believe that since they just built a much more powerful engine that you have to run a much richer A/F ratio or starting throwing more jet at it. More times than not (assuming good choices were made during the build for compression, squish, cam events, etc) the engine will need LESS timing and fuel to make it happy.

The other thing WAY too many people still do is to eliminate vacuum advance for street driven engines. It is a load sensing device and does nothing more than add a few degrees at very light engine load so you can use less fuel and effectively make optimum power with a leaner mixture. Only positives come from that deal, and i use VA here on all but the most powerful high compression full "race" engines.

I'll finish with a quick story as to how that works in the real World. A few years back I was hired to do a complete carburetor rebuild/restoration for a 455 Olds W-30. The car had been modified and raced for decades and the owner purchased it and was returning it to "stock". Well dumb-ars me didn't ask enough questions about the build but I was insured compression would be stock and the located a correct Olds W-30 camshaft for the build.

So I finished up the carb and tested it here, absolutely FLAWLESS as it should have been beind a factory "hot-rod" carburetor. I actually put it on my own 455 powering the Ventura at that time and ran the bag off of it, testing it intimately in every single area. I've never tested one that worked any better. So I ship it out and almost immediately the customer calls up bellyaching about it not working well. Says it idles perfectly, full control with the mixture screw and perfect anytime he goes to heavy or full throttl with it. The problem he's having is that it hesitates right off idle at very light engine load and "surges" slightly at cruise, plus "crackling" int he exhaust when he's driving very gently. (All symptoms of being a little lean in that range).

So he sends it back, and once again it's FLAWLESS here, so I call up and ask a few more questions. He insures me the cam was degreed correctly, squish and compression are right, and his initial and total timing are fine. Then I asked him how much vacuum advance is added? Vacuum adance? We (his Olds "guru" engine builder) and himself took that "crap" off WAY back in High School when the first started owning/driving these cars. He said he tossed the POS factory points distributor out and put in an MSD Billet distributor just like ALL the other muscle cars he's owned.

Well, I guess trying to discuss the need and advantages of VA aren't going to appoly here so fattened up a PERFECTLY tuned carb at light part throttle and sent it back. It was too rich and I lost that "clean/crisp" throttle response when tested on my 455, but it solved all the issue and the customer was happy with it.

I have many hundreds of stories just like that one, but at the end of the day what needs to happen is that the customer is happy. I've learned over many years of doing that that I'm simply NOT always able to educated them or help them make things as good as they can be..........but rest assured if/when you hire me to do some work I'm going to ask about your distributor and whether or not you are using VA or not!..........

PS: I did make a few minor spelling errors and one mistake in the previous response. Down where I'm describing how much timing the distributor needs to be set up for it should say:

The cure is simply to modify the mechanical curve for 10 degrees of advance (20 at the crank)

[ta man]

Even with the most thought out and ideal combination the end user must know some basics to tune ..or have someone who knows how..quite often it is just a minor screw in or out of the idle mixture screws or up and down on the idle speed.