Rich Idle AFR reading

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stumblinhorse

stumblinhorse

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I will run through my set up real quick.

69 318 stock bottom end, 100k miles. Stock cam, timing chain etc.
Rebuilt heads (675s) comp springs 901s. Stock Lifters.
Weiand Stealth
Edelbrock 600 primary leaned out 2 jet sizes (.095) Rods and Secondary leaned out 1 size.
Hedman Long tube headers to 2 1/2 duals, no cross over, borla mufflers
Stock Box Mopar electronic ignition 12* initial and 34* all in at 2500 rpms.
Idle is set at 750 RPMs
It is a manual transmission.
I live at around 7k feet above Sea level.
AFR gauge is a wide band AEM on the even side collector.
Cruising numbers are 13-15:1 and mileage is way up to 13.
Idle after warm up is 11:1 off idle get right into 13:1
WOT is about 12:1, not really spending a lot of time there :)

After I finished putting every thing back together I used just the AFR gauge to set idle mixture to be about 14:1 on the gauge. It would go way to lean off idle and stumble, would go "---" at over 17+ AFR. So I pulled out the vacuum gauge and set the IMS to be highest vacuum, 14" on vacuum. That results in about a 11:1 AFR reading.

I am just learning and I know there is a lot of moving parts in getting this correct. So where should i look? I can lean out the IMS a little and sacrifice some vacuum. No power brakes so no problem there. But shouldn't highest vacuum be where the carb wants to idle. So is my rpm too low? Just lean it out and raise the rpm a little to see if the vacuum comes up? Or should I be looking at timing? At around 10* advanced I get some burbling on decel. So I bumped it to 12* and that went away.

Thanks for any help.
 
I am probably not the best in interpreting any A/F device, but I would never use any device to adjust idle. Idle adjustments are too personal, too unique to each motor. Idle speed, fuel mixture are a somewhat delicate balance that an old pro can "hear" if it is right. Idle speed is determined by the lowest comfortable speed not allowing the motor to die. Mixture is a balance from side to side (2 corner) and sometimes front to back (4 corner idle). It usually is the leanest that an engine will consistently idle at, and I usually go a little richer from there. I don't care about vacuum readings, that will take care of itself,. It is always more important that the motor is happy. I think that all the numbers you provide are fine. Be careful how you interpret them. A/F can go lean off idle if you are moving the throttle slowly. The carb is transitioning from one circuit to another. Under normal use you would hit the throttle harder engaging the accelerator pump which compensates for this dead spot. Hope this helps to begin the discussion.
 
For your elevation, your first numbers were pretty close in my opinion. I am at 1000' and hot idle 12.5-13. mild cruise is 13.9-14.5 and when the PV and secondaries kick in it goes to low 12's.
Stoich is 14.6 and there are charts available to give you the performance at different ratios. I like to keep my car as close to 14+ as I can. No leaner with my cam and timing though!
 
stumblinhorse said:
I am just learning and I know there is a lot of moving parts in getting this correct. So where should i look?
Click to expand...
Look at the general AFR curve in this post: Wideband

stumblinhorse said:
I can lean out the IMS a little and sacrifice some vacuum. No power brakes so no problem there. But shouldn't highest vacuum be where the carb wants to idle.
Click to expand...
Basically yes. Highest vacuum at idle (in gear with an automatic) is a good indicator of power. With a manual transmission, just set it a tad richer than leaner because obviously you can't play with the idle in gear.

stumblinhorse said:
So is my rpm too low?
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750 rpm is actually a little higher than stock. The aftermarket intake is the only thing that might make it a little less efficient at idle - and therefore need the slightly higher idle speed.

stumblinhorse said:
Just lean it out and raise the rpm a little to see if the vacuum comes up?
Click to expand...
I see no reason to raise the idle speed.

stumblinhorse said:
At around 10* advanced I get some burbling on decel. So I bumped it to 12* and that went away.
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So here's the deal. The LA 318 was born in the begining of the smog control era. A '69 would have been set to idle at 14.2 AFR with timing around TDC (Dodge) or 5 ATC (Plymouth). In my opinion, for what you are doing - looking for performance - the initial you are using is about as much advance as it probably needs. It should be able to run decently at 600- 650 rpm with as little as 5 BTC (and the AFR closer to 13:1 than 14:1).

In my opinion, a stock cam 318 is more efficient at low rpm than say a 340 or a 440 A134 and therefore does not need or like too quick of an advance.
My suggestion is slow down the advance. Make it look something closer to the non-CAP 318 timing. Doesn't have to start at 5*, 10* or even 12* would be OK, but the overall shape should be closer to factory non-CAP 2bbl 318, or a 4 bbl 273.
blob-png-png.png


1967-273-4bbl-PP-Timing.png
 
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If you notice the AFR changing with timing, just be aware that's not quite real. AFR at idle will change with manifold vacuum (stronger signal = more fuel) and throttle position. Apparent AFR changes on the meter from timing alone are because of the meter interprets oxygen left over and therefore is not always correct about the fuel and air mix going in when the combustion doesn't complete exactly as expected.
 
I agree
When you bring in that much advance, then to keep the idle speed down, you have to close the throttle. This reduces the fuel flow from the transfers, and so you have to richen up the idle mixture screws in compensation.
Then when you slowly tip in the throttle, she goes momentarily lean until the transfers come back on line, and then she goes rich, cuz the mixture screws are set fat. To cover the resulting hesitation, you mess with the accelerator pump, making it better for a moment, but then worse.
The cure for all this is to synchronize the transfer ports to the mixture screws and let the idle-timing be what it will be, to get the rpm you want it to be or close. But after the T-ports are synced, don't mess with the speed-screw any more. If you want to change the idle speed, you do it with idle timing.
As to the power timing, it should fall between 32 and 36, once the Rs are up to maybe 3000 to 3500. The rate of advance can be tricky with a manual trans, but with a stock cam 9.2 compression engine, I suspect all-in by 2500 is too fast.
If you look on Mattax's kindly supplied graph, you see curves with kinks in them . I like those for manual transmissions. Altho I like the kink a little later and the all-in a little sooner.... than those shown, but it really depends on the combo. Hiway gears and heavy cars could be more like the ones in the graph. But a lightweight-A and 3.91s, could have a later kink, and a shorter run to all-in.
If you get it right, you should be able to run cheap gas on full timing of about 35* +/- 1*.......WITHOUT detonation.
I am no fan of setting the timing to obtain the highest rpm or vacuum, with a carburetor, AND A MANUAL TRANS. Almost invariably, this leads to driveability issues. Set the T-port sync, and things will start to fall into place.

Burbling on decel is likely a two part issue;
first is a lotta gas in the headers, and second is oxygen also getting in there. This continues the burn in the pipes, and you hear it as burbling. Advancing the timing was a bandaid, that started the fire early,to try and finish it early, so less raw gas made it into the header, from the heavy draw on the too-far-open mixture screws, and too-far closed throttles.
The cure is to reset the mixture screws to factory spec,(maybe 1.5turns out) syncing the transfers,which will reduce the closed-throttle decel-vacuum, and sealing the headers;both ends.

The idle circuit HAS to get fuel from the transfers. The mixture screws should be thought of as a trimmer.

You can set the transfer slot exposure to something like .060. Then set the idle speed by setting the timing to something like 8*;whatever it takes to obtain about 700 rpm idle.. Then trim the idle quality with the mixture screws. You might find the best screw setting to be 3/4 to 1 turn. OR
Or you can reduce the exposure to say .050, add 2* timing, and reset the mixtures to 1.5 to 2 turns.... OR
Or you can reduce the exposure to .040, add 2 more* timing, and reset the screws to 2.5.
The engine may idle equally well on any of these settings..... because it is getting enough idle fuel.
But it will like just one of them, when you tip the throttle in. It will tell you when the exposure is too little, by hesitating. This shows up on the AFR, as a lean dip, then rich.
It will tell you when it is too rich, by going mushy. Find the two points and split her up the middle, OR
Pop the carb off, flip it upside down, find the T-ports and adjust the curb-idle speed screw to make the exposure slot square. Then bolt it back on and do not touch the speed screw again. Use idle timing to adjust the idle speed. Then trim the idle quality with the mixture screws.Finally synchronize the Accelerator pump, and lean it back to factory specs. If the mixture screws come in, in the center of their range, then you are really close to perfect. Make a note of their adjustment setting, AND the rpm.
Now, using the fast idle cam, set the rpm to around 2000, exact speed not important. Make sure your Vcan is hooked up, and working. Figure out if the mixture is rich or lean. I do this by bringing a bunched up shop rag over the airhorn... then adjust the mixture screws to obtain the highest rpm, with the mixture screw setting within the limits of 1T to 3Turns. If it's not within those limits, then the low speed circuit may need work. But if it falls in that range, Then
Release the fast idle cam, and allow the engine a minute to cool. Now reset the curb-idle screw up to 1/2 turn in either direction to readjust the idle speed close to it's former speed.
Now go for a low-rpm driveability test; from idle to about 2400, and up into second gear. Use the gas pedal, tipping it in gently, looking for the hesitation.
With a stick car, this hesitation is extremely annoying; you'll have no trouble finding it.
If this is successful,in having no hesitation, record all your settings.
Then check your power timing, and rework your Distributor to get the 34 to 36, or whatever it takes to keep it out of detonation at about 3600rpm and over. More is not better. On the street you will not notice if you are 2* less than optimum. But your engine will complain about just 1* too much.
Finally bug out the rate of advance.
After that you can attack the Vcan optimization.
 
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I am an AFR gauge user and I have to agree with the statement from Murray " Idle adjustments are too personal, too unique to each motor."

If my AFR says 16.5 and I like the sound and characteristics of the idle then guess where it stays.:D
(It's actually in the high 14 range at 700 RPM, but the point is still the same)
 
@Mattax great info, thanks. My FSM show 5* btdc for factory 2bbl timing and for altitude to advance up to 5* more. So you think going back to stock timing even with my electronic ignition? I will give that a try. Also setting IMS according to rpm not total vacuum. I will do that as well.

I will set idle speed to 600-650.

I will try to map out how quickly my timing curve is. Should that be with or without the vacuum line attached? My distributor is adjustable with different springs.

I will try all of this as soon as it stops snowing!

Thanks for the help.
 
Your engine is stock right; just bolt-ons. The cam,compression, and heads are factory. Those are the parts that have the biggest influence on the timing requirements.

The electronic ignition is nothing more than a maintenance-free trigger. It does exactly what the old points did, except wear out, bounce,leak voltage, oxidize, or weld up

Again; set the T-port sync. Then use timing to set your idle-rpm, be it 5,10 or perhaps 12 to 15. The more you give it now, the faster it will idle. Shoot for 700 with a stick; it's harder to stall....... unless you have a truck with a granny gear,lol, then 600/650 will be fine.
 
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stumblinhorse said:
My FSM show 5* btdc for factory 2bbl timing and for altitude to advance up to 5* more. So you think going back to stock timing even with my electronic ignition? I will give that a try.
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I think in general the factory engineers spend considerable time and testing to discover the timing for best power and efficiency. The service departments were given some latitude on timing based on customer and fuel quality available. The main differences in timing between mopar electronic and single point distributor will be the at high rpm, the electronic will lose some timing. If the advance is maxed out by 3000 rpm, you might notice 2-4 degrees less timing at 6000 rpm.


stumblinhorse said:
Also setting IMS according to rpm not total vacuum. I will do that as well.

I will set idle speed to 600-650.
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Do what seems to run best. Its just that opening the throttle too much at idle should not be needed and the engine should run nice fully warmed up at 650 rpm. You're close. When the initial timing is increased (10* to 12*) the rpms in neutral will go up. I'd suggest when the engine is fully warmed after a drive reset the throttle to 650 or 700 rpm, timing at 12*. Then adjust the idle fuel mixture for best vacuum by turning the screws in (leaner) until the rpm or vacuum drops, then richer until it just returns and then another 1/8 or 1/4 turn more (richer) depending on how sensitive it is.

stumblinhorse said:
I will try to map out how quickly my timing curve is. Should that be with or without the vacuum line attached? My distributor is adjustable with different springs.
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Plug the vacuum line. Always set initial and test mechanical advance without the vacuum advance. Is this the Mopar Performance distributor made by Mallory?

stumblinhorse said:
I will try all of this as soon as it stops snowing!
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Crazy late spring for everyone this year - but at least we're done with snow and salt here.
 
Carter type carburetors generally have two restrictions in the "idle circuit" so I don't get quite as concerned about being in correct range of transfer port exposure as a Holley 4150 type carb. That said, AJ has described why too high of an idle speed - if obtained by opening the thottle more - messes up the fuel delivery.
 
Mattax said:
Can't help on that one. When you have it open, take a photo of what advance mechanism looks like.
Click to expand...

Thanks for really helping out. The curves and instructions are located here:
https://static.summitracing.com/global/images/instructions/sum-851003.pdf
Quoting:
The factory installed medium blue springs in this distributor, generate a performance ignition advance curve that typically begins at 1200 RPM and generates 22-24° crankshaft advance; that is fully in by 3200-3300 RPM. This mechanical advance curve will work in most street performance engines.
 
stumblinhorse said:
Thanks for really helping out. The curves and instructions are located here:
https://static.summitracing.com/global/images/instructions/sum-851003.pdf
Quoting:
The factory installed medium blue springs in this distributor, generate a performance ignition advance curve that typically begins at 1200 RPM and generates 22-24° crankshaft advance; that is fully in by 3200-3300 RPM. This mechanical advance curve will work in most street performance engines.
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With 12* set at idle of 750 rpm, it will still be 12* at 650 or 700 rpm. That is good. Unfortunately you may also find it is still at 12* all the way up to 1200 rpm. However, that description doesn't match what you found, which was it hit 34* by 2500 rpm. If the advance did what is in the instructions, it would be:
12* at 750 rpm,
12* at 1200 rpm
34* at 3200 rpm (12* + 22* )

There's not quite enough info in the instructions for me to be sure of what's inside. The advance degrees adjustment is similar to a Mallory YH mechanism. However the springs offered are similar to a Delco mechanism. With a YH mechanism, changing the the slot length (degrees of advance) changes the initial spring loads. The little graphs don't help because 3 of them use a different amount of advance than the others. I prefer not to guess, or go on and on about different types of advance mechanisms, the limitations and how each of them may be adjusted.
 
Mattax said:
With 12* set at idle of 750 rpm, it will still be 12* at 650 or 700 rpm. That is good. Unfortunately you may also find it is still at 12* all the way up to 1200 rpm. However, that description doesn't match what you found, which was it hit 34* by 2500 rpm. If the advance did what is in the instructions, it would be:
12* at 750 rpm,
12* at 1200 rpm
34* at 3200 rpm (12* + 22* )

There's not quite enough info in the instructions for me to be sure of what's inside. The advance degrees adjustment is similar to a Mallory YH mechanism. However the springs offered are similar to a Delco mechanism. With a YH mechanism, changing the the slot length (degrees of advance) changes the initial spring loads. The little graphs don't help because 3 of them use a different amount of advance than the others. I prefer not to guess, or go on and on about different types of advance mechanisms, the limitations and how each of them may be adjusted.
Click to expand...

Thanks, I will play with it tonight. Appreciate your help
 
I read your issues as being the transition circuit not being right or the ignition being weak. No way that you should (or will) have such a big change in AFR going from idle to just off idle'. As said, once it starts to misfire, then the AFR readings are going off kilter; the free oxygen in the exhaust rises and that gets falsely interpreted as a lean condition.

You're having to richen up the idle mixture to cover either a lean misfire or ignition misfire condition just off idle. Working on the advance alone is also just covering up this issue. Find out what is wrong in the carb 1st OR in the ignition energy being produced. If the spark is on the weak side, then it will have a harder time firing through the pressure changes in the chambers. And that can apply to the plugs you are using; we don't know what you have for plugs and wires or how good you ignition really is. Putting in an HEI might be a very good next step for you.

FWIW, if you hit in the 15 AFR range at light to moderate cruise conditions, don't worry over that one little bit. It will be just fine and, with your engine, going into the 16 range at light cruise wouldn't bother me either, my son and I have his high compression 340 in the 15 range all the time on the interstate... not a hint of detonation ever. Fuel economy at cruise will go up quite a bit when you properly abandon the idea of wanting stochiometric AFR under cruise conditions, and it won't hurt your type of engine at all. As a pint in fact, the Lean Burn system pushed cruise AFR up into the 18 range.
 
I have been playing with all different idle mixes and never got it to where I want. I retarded timing to 5 btdc and then back up to 10 and still didn’t get it right. I switched to manifold vacuum to see if that would make a difference and still rich. So I picked up a fatter metering rod and put that in and now it is impossibly rich!!! At all RPMs. So I obviously broke something. I am just one size under the fattest rod and my carb is set at the #27 reference point in the manual for a 1405. Which is the leanest setting in the manual. I am thinking about a fuel pressure regulator, since all of a sudden after a simple rod change I am rich across the entire rpm range. The carb has only a 250 miles on it and was just rich at idle, now cruising etc I am 11-12 on the afr gauge. And I can only get to 15-16 on deceleration. I put back the old rod and the stock rod and both seemed to give same rich readings across the board.

So here is my plan, fuel pressure regulator set at 4.5 psi and move up to from 12 to 14 heat range champions. And start again. Very frustrated, thought the rod would do it.

Since at 7k feet above sea level I can only get 14” of vacuum, should I also change step up springs to something lighter? Not sure that will change anything. Btw engine runs fine has always ran fine, no real symptoms, except now I can smell fuel at wot. So whatever happened with a rod change knocked everything out of whack.
 
Make sure that your fuel level is correct. Then
Get your T-port sync dialed in, then
leave your speed screw alone.Then
Adjust your idle speed with timing and idle-air bypass and nothing else.
Your combo should not need any idle-air bypass.

The mixture screws are to fine-tune the idle quality. Forget trying to tune the idle with the AFR gauge.
If the mixture screws cannot get the quality of idle that you expect with the screws in the center or near center point of their adjustment..... THEN you can adjust the speed screw up to 1/2 turn, whichever way it needs to go as revealed by where the mixture screws are, and then reset the mixture screws to their centerpoint +/-, 1/2 turn. This is not an idle speed adjustment now!! This is a transfer fuel adjustment to get the mixture screws in the zone.
Idle speed is still adjusted with timing and idle-air bypass and nothing else,as above.
With a manual trans, don't be afraid to idle it up near 700/750. By the same token, try not to run more than 16/18 initial. I'm too lazy to go search what cam you are running but the 14" idle tells me it's pretty tame, and so it doesn't need a lot of initial timing. Run the minimum amount to get your target idle timing with the T-port sync correctly set.
Don't forget to limit the full-load timing to 35ish; whatever it takes to keep it out of detonation. At 7000 ft,you may have to delay full timing to a higher than usual rpm.

But before you do anything check that your jets( all of them)are still screwed down and that the metering rods are undamaged and properly located. Take the springs right out, to set the lowspeed running ...if you have to
 
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Well, I think I figured out what made everything to go rich. My new alternator that I put in, is actually dead. So my battery is almost dead. I was noticing my timing light was really dim and checked the battery and it was right around 12V when running. So I am guessing I am not getting much fire...

I am going to fix that, again. And start again.
 
Hope it is that easy! If the meter/AFR controller gets much less than 11 volts, then the AFR METER itself can start going out of whack.
 
Got my old alternator tested and output was fine. So ordered a MoPar constant output voltage regulator and a better multimeter. I am thinking or hoping that solves some of my problems with being rich, just too low of power to the ignition. My electronic ignition is getting at best 12v of power from my battery. Hopefully Thursday night I can fix it all up when all the parts show up.
 
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