AFR misleading? Plugs tell a better story?

Fuel and Air Systems

  1. skidmark

    skidmark Well-Known Member

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    Thought I was OK watching AFR readings, Idle AFR 13.0 (Idle was set by highest RPM method). Cruise 40-50 AFR 14.5. Then pulled spark plugs, rear engine plugs 5,6,7 & 8 are leaner/whiter than the front.
    Trying to tune carb and fix a miss/weird stumble at idle. Engine runs smooth once RPM’s are increased, also seems to use a lot of gas.


    When searching here I found a thread where AJ mentions manifold vacuum port at #8 intake runner, sure enough that’s were vacuum hose for brakes is connected. Seems like this could contribute to the problem?

    Will move brake hose to rear center port on carb.

    Also read about needing a different PCV valve with lower vacuum? Vacuum is 11.5 at idle.

    Anyone have any input on why rear plugs may be leaner?
    What about different PCV valve?
    Any input is welcomed.


    72 340 stock bottom end
    Iron heads with new valves
    Air gap intake with cool carb spacer.
    670 street avenger #68 primary jets
    Idle screws out 1-1/4 turns, idle speed 800-825.
    Idle vacuum 11.5”
    Hei with stock distributor and e core coil, 18 initial, 34 total.
    NGK GR4 plugs gapped 44
    Hughes whiplash cam.
    4 speed, 323 rear

    56A1B7B5-7EC9-447B-9387-D2BC6C2A9711.jpeg 371C0F8B-AC8E-4633-8973-FB5C6A76322A.jpeg 4059A4DD-4A05-49DC-9506-A355DB040D39.jpeg
    98181B53-3D50-4EC6-A0BE-D52EBF5D3195.jpeg
     
  2. BigBlockMopar

    BigBlockMopar BigBlockMember

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    Unless you put 8 O2-sensors in every port AFR reading are always an average of 4 or 8 cylinders.
    Start with plugging the brakebooster vacuumline into the correct port and do your readings after some time.
     
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    • TT5.9mag

      TT5.9mag Two atmospheres are better than one

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      I think with that whiplash cam and it’s 107 lsa tuning idle with an afr gauge will be misleading. Learn to read the plugs carefully.
       
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      • DrCharles

        DrCharles Well-Known Member

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        I have read that big cams with lots of overlap allow excess air into the exhaust, resulting in a falsely high reading.

        My wideband shows the 451 is idling at 14-14.5 which I doubt is real given the size of the cam: 272 @.050, .652 gross lift, 107 LSA (in at 105), and the smell of the exhaust (not eyewatering or sooty, but definitely not stoich) ;)
         
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        • yellow rose

          yellow rose That is the definition of RETARDED

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          The PCV gets hooked to the back of the carb, not the power brakes.

          Run the power brakes off the intake manifold.

          Set your power valve opening by CRUISE vacuum not by idle vacuum.

          If you are lean at a cruise (can't see much from the pictures) you can use a smaller idle air bleed, but on your carb it may not be that easy to change.

          Unless you have 8:1 compression that plug may be too hot.
           
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          • TT5.9mag

            TT5.9mag Two atmospheres are better than one

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            Unless you have 8:1 compression that plug may be too hot.[/QUOTE]

            I’m going to venture a guess here but 72 340 with iron heads and a whiplash cam tells me 8:1 might be close. That’s what those cams are for. Low compression and “that muscle car idle”. Op were the heads shaved at all or do you know the chamber volume? Maybe some compression test readings?
             
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            • skidmark

              skidmark Well-Known Member

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              I’m going to venture a guess here but 72 340 with iron heads and a whiplash cam tells me 8:1 might be close. That’s what those cams are for. Low compression and “that muscle car idle”. Op were the heads shaved at all or do you know the chamber volume? Maybe some compression test readings?[/QUOTE]


              Heads shaved IIRC .012
              Don’t know chamber volume, pistons are below block deck.
               
            • TT5.9mag

              TT5.9mag Two atmospheres are better than one

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              See if you can get some compression test results. Can’t tell for sure but number 3 plug looks wet to me and may be a source of a “misfire at idle”
               
            • dano

              dano Evil Handy Man

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              You can foul O2 sensors as well. So if you ever ran pig rich, they can get sooted up. Clean with compressed air.
               
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              • mopowers

                mopowers Well-Known Member

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                When's the last time the AFR gauge was calibrated? Most of them recommend annual calibrations.
                 
              • skidmark

                skidmark Well-Known Member

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                Hopefully tomorrow for compression test.
                Agree 3 is darker, but not wet.


                Don’t remember running really rich. Maybe I’ll get another sensor.


                Just this last Friday, after pulling plugs.
                Same numbers with new plugs. Innovative AFR
                 
              • skidmark

                skidmark Well-Known Member

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                Close up picture of #3
                A48DB964-CD8F-4B32-9C82-442F62246639.jpeg
                some more
                94D6BC86-925D-4F68-8EBA-B11617B580A7.jpeg B738AF3F-1957-42A2-B746-E990DCD4EB96.jpeg
                 
              • skidmark

                skidmark Well-Known Member

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                273 rocker arms, probably should check preload? Couldn’t hurt?
                Hughes wants .080-.095
                 
              • RustyRatRod

                RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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                What's supposed to be the "optimal" air fuel ratio? 14.7:1 ain't it? That's GASOLINE and air. Not gasoline ETHANOL and air.

                Just food for thought.
                 
              • Aaron65

                Aaron65 Well-Known Member

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                Innovate's gauges are calibrated to show 14.7 when the engine's running at stoich, so even if you use E10 or E85, it'll show 14.7 at stoich. I called their tech line to verify this when I got mine. Either way, some engines just don't like running that lean; I've found that cruise in the 14-15 range and power in the 12-13 range will keep most engines running pretty well.
                 
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                • RustyRatRod

                  RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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                  And you bought that line?
                   
                • Biomedtechguy

                  Biomedtechguy Well-Known Member

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                  • Aaron65

                    Aaron65 Well-Known Member

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                    Well, yeah I did... :) Actually, I had been doing some research on the subject, and ended up coming to that conclusion on my own, and I called Innovate to verify. Oxygen sensors are designed to read in lambda, where stoich is labeled 1.0, and the gauge is then calibrated to convert lambda to the scale for straight gasoline.

                    Believe it or not...doesn't matter much to me.
                     
                  • Mattax

                    Mattax Just the facts, ma'am FABO Gold Member

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                    That is basically correct.
                    Lamda is the way it is always interpreted. Those who use lambda don't need to consider what type of fuel it is. Those of us who are so used to working in AFR find that difficult so convert it to a 'typical' gasoline, 14.7:1. Is it exactly corect for any one gasoline? no.

                    Does it matter? Like you posted, no. The lamda or AFR is just a reference point, not some target. The only times we care about the value is to know its ballpark for the conditions, or when its targetted for emissions reasons. For example, in the late 60's Chrysler spec'd the AFR for the idle at 13.9:1 or so and then later to 14.2:1 to meet HC and CO targets at idle. AFR was determined using a CO sensor system - which is better measurement tool for stable conditions like idle.

                    @skidmark I wouldn't go nuts over differences in spark plug appearances over a wide range of conditions.
                    At idle, with an Air gap, and moderately low vacuum from the cam, it will be difficult to get really good fuel distribution.
                    If you did some tests at steady part throttle - look at the plugs, then at least you're looking at just one situation.
                    Then do the same for a hard wide open throttle run or runs.

                    When the factories did this, they either made changes to the intakes or they altered the carb boosters to direct fuel toward or away from cylinders as needed. You can see this latter technique used in the Direct Connection bulletins for carb setups for drag racing.
                     
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                    • Mattax

                      Mattax Just the facts, ma'am FABO Gold Member

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                      Does the stumble go away with a 1/4 turn more, or a 1/4 turn less of the idle mixture screws?
                      What is the timing at at 850 rpm? and what is it at 950 and 1050?
                       
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                      • AJ/FormS

                        AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                        >The PCV needs to feed into the primaries and dump in the vicinity of the idle discharge port and transfer slots. This promotes good mixing with the low-speed circuit.
                        >The vacuum booster should attach at the secondary side port, and you must prove that the booster is not constantly sucking air.
                        >The port on the intake is for a Vacuum Tee to supply the A/C system, or the EGR amplifier, etc.
                        >You need a 10.5PV. Don't pay anybody no mind, just install it. 68s on the primary side is probably too lean, but wait until the 10.5 has been tested.

                        I ran a Hughes HE2430AL in my 11/1 367 for 4 years. It has a very similar intake lobe to yours, just retimed. It ran on MJs from 70/78&10.5, to 72/80&10.5
                        >My cam liked some idle bypass air,I think yours will too.My Ica was 63 compared to your 57, and my overlap was 53 compared to your 62,so you will probably need some. My 750DP liked the float set on the low side of the window.
                        > You must get your transfer slot exposure under the throttle plates,set to; from square to slightly taller than wide. If that means just 14 or 12 degrees idle-timing, then make it so. It's a 4-speed car, you cannot time it like it was a hi-stalled automatic; 14* is already lots. (Well you can, but the engine won't be happy about it; oh sure it will idle nice; but so what, if stumbles on tip-in)
                        If you do this, you may have to change your power-timing back to in the range of 34 to 36. The all-in can be later than usual because of the manual trans, you won't hardly notice the difference.
                        >Your cruise timing can be fierce. Numbers in the mid 50s are common.
                        > If NOT running a CDI ignition,then,get rid of the E-core coil in favor of an Accell Super, the big square-top one; it makes a waaay hotter spark on the same input voltage.
                        > when correctly done, you should be able to idle her down to 700/750 and don't pay attention to the idle-vacuum. When you idle it down,to reset the transfer-slot exposure, you may have to add idle bypass-air and close up the mixture screws some. Right now, they are compensating for the fuel she's NOT getting from the transfers ........ because of the 18* idle timing. Get this fixed, reset your accelerator pump, and your weirdness will go away.
                        OK I confess, I got that 223/230 cam of mine to idle at 550 in gear and dragging itself around the parking lot.... with the 10.97 starter gear, which is 4 mph.
                        > my 223* cam did not like 3.23s off the line with the regular 2.66 low box. I solved it with the 3.09 Commando box, and 3.55s which I have run ever since. Now it's a blip-it and go deal.
                        Your starter gear is 2.66x3.23=8.59 pretty steep for a 340; compared to mine at 3.09x3.55=10.97, which compares to yours at 2.66x4.10=10.91. A starter of at least 10/1 is a nice easy first gear for even a low-compression 318. I'm not saying you should rush out and get these. But, if your '72-340 is a low-compression engine, the 62* of overlap is not gonna play nearly as nice with what you currently have. Someday, at least get the Commando box, cuz downshifting into 3.09x3.23=9.98 starter at 30 mph, is a wicked fun thing to do.
                        If you get a chance to swap carbs, a DP is the bomb.
                         
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                        • yellow rose

                          yellow rose That is the definition of RETARDED

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                          Why do you want to pull the PCV off the primary side? I'm not familiar with that particular carb, but most any Holley from the beginning used the rear port for PCV and a port in an intake runner to power the booster. Just curious as to your line of thought here. Not saying you are wrong, just want to work through your thinking.

                          OP, between Mattax and AJ they have pretty much nailed it. You dang sure need a 10.5 power valve and probably a 12.5 if you could get one (when I get caught up I'm going to start playing with power valves to see if I can get them to open sooner than 10.5 because I could use it for sure) and I'm waiting for AJ's thoughts on how he wants to plumb the PCV valve.

                          What Mattax said is equally important. You must tune to the leanest cylinder and when you have that style of intake, especially at low engine speeds you make air and fuel (which do not weigh the same and may not be homogeneous) turn too many tight corners. When this happens, the air can make the turn but the fuel has trouble. And the fuel falls out of the air (for lack of a better term due to my limited education and vocabulary), and becomes what is called wall flow. That is fuel that has separated from the air column and is now liquid running down the port walls and floors.

                          Most all of that fuel never gets back into the air stream and goes into the chamber and adds to stratification issues.

                          And you end up with what I believe we see with your plugs. The plugs are both looking a touch lean and also a bit fat.

                          At this point, you can tune to the leanest cylinder and let it go, or spend some time trying to clean up the distribution issues you have.

                          I suspect this engine would respond well to using a burr finish on the intake manifold runners. Not to make them bigger, but just to try and get the wall flow issues under a bit better control. Just guess on in this as I've never been able to duplicate the correct burr finish (or what I believe to be the correct burr finish) so I don't do it. But that doesn't mean it doesn't work. Also, not sure how much work like that you can do with an intake of that style without taking forever to do it.

                          Just my spit balling.
                           
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                          • TT5.9mag

                            TT5.9mag Two atmospheres are better than one

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                            The misfire at idle is making me wonder what is wrong with the wire or cap on number 3 cylinder. That plug looks enough different than the rest that I would focus on it.
                             
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                            • yellow rose

                              yellow rose That is the definition of RETARDED

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                              I agree. Could even be a bad plug. But it should be corrected first.
                               
                            • mbaird

                              mbaird mbaird

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                              Great thread
                              I have been chasing a similar issue with the Barracuda engine .
                              Plugs look lean but eyes say rich ! AFR is all over the place
                              Run both a Slayer Holley 750 VS and now a new 650 AVS-2 ... engine blubbers a bit up to 2000 rpm... recurved the dizzy 100 times ...changed rods and jets lord knows how many times !
                              Starting to think the Cam is crap grind.
                              Got a ton of cylinder pressure (170ish )
                              11” vacuum at idle , 15-16 @ cruise .
                              Mileage sucks.
                               
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