AFR misleading? Plugs tell a better story?

Fuel and Air Systems

  1. AJ/FormS

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

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    naw, if the lobes are fine,that cam is fine. With 170psi pressure, across the board, all you need is to stumble on the right tune.
     
  2. 512Stroker

    512Stroker We are all here because we are not all there.

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    Those plugs are lean
    Pay attention to the timing mark on the plug ground strap, it appears a bit to close to the plug threads.
    Reference this for info on reading plugs
    Spark plug reading can be complex and sometimes frustrating task this page will help make it easier and the results rewarding
    Good luck
    Keep posting it's great stuff
     
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    • AJ/FormS

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

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      The short answer is;
      Reason #1; my experience;
      Every time I have run the PCV to the secondary side,on a dual plane street engine, that requires idle bypass air,and does not have a 4-corner idle; ( about 99% of my tunes,lol, I get idle-tuning grief, which is right away much reduced or gone when I reroute that hose to the primary side.
      The longer answer;
      My conclusion was that the 6 and 7 cylinders were idling lean. I don't idle-tune with an AFR, and for me reading the idle tune on the plugs is hard. But I can darn sure tune by the hesitation or stumble on Tip-in. In my personal combo,I like to run as little pump-shot as possible, cuz you know, I'm always trying to drive further on less fuel, but my right foot doesn't co-operate so much. And because it's a 750DP, so every lil bit helps. So then I am forced to work the tip-in differently.
      When the combo is an automatic with a hi-stall, this is less of a deal.
      But with the OPs 3.23x2.66 manual trans starter gear, the sub 2000rpm zone has to be tuned pretty tight, cuz 2000 is still only about 18mph. I can't be lazy. With 10/11 to 1 starter gear, I could be, but everyone comes with small street gears.
      I only tune SBMs, and all the cams I have tuned run between about 220 and 240, (Except two were bigger), because that is all I know, and they are in my comfort zone, and I get paid, (OOPs used to get paid; now semi-retired) to get it right. And this leads to;
      reason #2, the reward
      When I get it right, the smiles and hugs are worth so much more than just cash.
      xxxxxxxxxxxxxxx
      As to the plumbing the booster;
      I have tried several times to run the booster off that rear runner, and obviously it will work...... to a degree.
      But I have found that there come times and situations when the booster (not all boosters) is being manipulated during braking, and the one runner cannot keep up. Since the OPs carb has all the ports he needs to save aggravation, I jumped on it.
      Yeah, I know, that was my thinking too; pressure in the intake should be equal in all directions; and in the plenum maybe it is. But in an individual runner with a performance cam, the moving piston is wreaking havoc in the runner. So I reasoned that the vacuum in the plenum, might be just an average,but that when the piston connected to that runner is driving fuel-charge back up into the intake with the late-closing valve still open, well I had to put a vacuum gauge on there. After that, I have never again used that runner. It might work for some guys with some boosters, with some combos; But not for my combo with my aggressive driving style. It seems to work just dandy fine when you rev it up, and the vacuum rises. So if you have big-number rear gears, I suppose it would be just fine.

      Those are my thoughts; I too could be wrong...
      But no one has yet complained, so I got no feedback data ,lol.
       
    • yellow rose

      yellow rose Doctor of Thinkology.

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      Ok. That makes sense now and that's why I asked. I figured I'd ask.

      I haven't used a carb without 4 corner idle since sometime in the mid 1980's.

      So that makes sense and the OP should try it.
       
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      • mbaird

        mbaird mbaird

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        Do you mean it is not losing a lobe ?
        My thinking was it may be a poorely thought out grind.
        It idles like a big grind but doesnt pull up high like a big grind . Kind of dies off around 5k... I-should have gone with the 800 AVS2 but I dont think that is the whole issue with the top end.
         
      • DrCharles

        DrCharles Well-Known Member

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        Interesting. That is exactly the setup I have... except that I drilled all four plates 3/32" so I could set the primary and secondary transfer slots both "square".
        Just wondering what kind of grief you are having?
         
      • twin 640

        twin 640 Member

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      • twin 640

        twin 640 Member

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        Big overlap cams are for big compression also that cam is a single plane big rpm intake manifold.keep an eye on those plugs for white
         
      • 512Stroker

        512Stroker We are all here because we are not all there.

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        Just my 2 cents, but you may be running out of fuel on the top end.
        What fuel pump are you using? Fuel pressure at rpm?
         
      • mbaird

        mbaird mbaird

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        Fuel pressure was actually too high so I installed a regulator.
        Multiple carbs has not changed the engine characteristics by much other than the AVS-2 is much cleaner down low. But still has a bit of burble below 2k
         
      • 512Stroker

        512Stroker We are all here because we are not all there.

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        Do you have a fuel pressure gauge mounted where you can see it from the drivers seat at speed?
         
      • skidmark

        skidmark Well-Known Member

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        Took carb off to check T slot.
        Any thoughts on Cool carb spacer, 4 hole OK or open up to match plenum openings?
        AE898877-85CF-4404-B662-130CC6FD3038.jpeg 780163D0-98F8-4BE0-BF32-9924C786CE5B.jpeg
         
        Last edited: Feb 5, 2020
      • skidmark

        skidmark Well-Known Member

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        Compression test, engine cold, all plugs out, carb removed. Did each cylinder 3-4 times, test numbers varied so I averaged. Some cylinders repeated better than others.
        1> 168.... 2>152
        3> 170.... 4>163
        5> 151.... 6>154
        7> 163.... 8>163
         
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        • skidmark

          skidmark Well-Known Member

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          0F53F904-3255-4A8D-BEAB-917ED1CB57F6.jpeg Set T slot so it’s taller than square.
          Picked up #70 MJ’s and a 10.5 PV.
           
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          • mderoy340

            mderoy340 Well-Known Member

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            Thumper/whiplash is for sound and that means overlap and reversion at low rpm. Dual planes require split jetting to even cylinder feed if you really want to even AFR to each cylinder. That means the idle, cruise and WOT may require tweaking. Also when reading plugs you need to look at the paired cylinders. On an airgap it's 1&4, 2&3, 5&8, 6&7. Follow the runners and you can figure out the pairs.
             
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            • TT5.9mag

              TT5.9mag Two atmospheres are better than one

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              The comp numbers look ok to me not great but ok. Warm test might show different. Any weird carbon tracking in the cap? If I was tuning this I would get wot afr squared away where I wanted it prob around 12.5:1, get the cruise afr pretty lean 14.5:1 and just deal with whatever that cam delivers afr wise on idle. Like mderoy340 said that cam will have some reversion at idle. Set the mixture screws for the best idle vacuum and live with it. All of this after finding the source of the misfire.
               
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              • AJ/FormS

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

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                With the carb off; cranking for 3 or 4 "times" meaning compression cycles, is probably ok. But for future reference, the cranking is done for as many compression cycles as it takes to get two consecutive same or near same psi readings. It might take only 3 or 4 cycles, but could take many more.
                EDIT;
                But if you mean that you tested each cylinder to max reading, 3 or 4 times, and then averaged and recorded those readings, well then ignore the above paragraph.

                With your pressure results; if you throw out the high, and the low, then the rest are running reasonably good;
                The problem I see is the huge 11% variation.
                I would immediately follow this up with a leakdown test, after oiling the cylinders,cranking it to distribute the oil, then pumping out the excess. When you perform the test, let the pressure stabilize at the largest leakage. The pressure will blow the oil down thru the ring gaps and the pressure may fall for a minute. The test needs to be done with the pistons at the top, to be the most accurate, and the more base pressure the better.; But use the same pressure for all cylinders. Most testers are regulated at 80psi, which is fine. With numbers as far apart as yours, I would consider the LD test as mandatory.
                Pay particular attention on the cylinders with the lowest pressures.
                For your test, I would take the rad cap off.
                After you have set each piston at TDC compression, be sure to remove your tool off the crank-bolt!
                If the rings are loose, the piston may not like to stay at TDC when you hit it with 80psi. In my tester I have installed a simple valve so that I can slowly let the air pressure build.
                Best of luck to you
                 
                Last edited: Feb 6, 2020
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                • skidmark

                  skidmark Well-Known Member

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                  To clarify, ran starter till cylinder pressure peaked and stopped rising. Did this at least 2 times on first pass. Numbers bugged me, so I did all 8 cylinders again.

                  These heads only have about 450 miles on them. I know that doesn’t mean they were done right or haven’t gone bad. Two machinist, lost confidence in both. That was a fiasco getting theses heads done. Thought of buying aluminum heads back then, in hindsight probably should have.

                  Ok leak test next.

                  EDIT: Just read your edit, that is correct.
                  Thanks for leak down testing info.
                   
                  Last edited: Feb 6, 2020
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                  • AJ/FormS

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

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                    The LD will give you a better idea of what's what; the heads MAY be fine, But I'll not hold my breath.
                    The pressure readings during the compression test are dependent on; the altitude they are done at,the ring seal,the valve seal, the head gasket seal, the cylinder integrity, and the cranking speed.
                    The LD test will tell us what and where the pressure is going.
                    If one assumes the two high cylinders are correct, than the others should be there as well. If you call 168/170 as 100% ok, then the lowest one being 152 is 89% of the highest, and your gauge saw three of those. The next grouping is 163/169=96% indicating about 4% leakage which is often accepted by other mechanics (not me) as tolerable. But with such low mileage, To me it is screaming " something is wrong".
                    It could be something as simple as the lifters pumped up making it impossible for the valves to seal,
                    It could be scratches in the cylinder walls,
                    It could be very hard rings not yet broke in,
                    It could be broken rings, or tapered/out-of-round cylinders, or upside-down rings,
                    It's too early to condemn the headwork.
                    But what if 163psi is correct? Then what? 152/163=93% pointing to 7% leakage.
                     
                    Last edited: Feb 6, 2020
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                    • skidmark

                      skidmark Well-Known Member

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                      The bottom end condition is unknown. Car ran fine when i bought it. Not a hint of any oil smoke. Then issues with heads. Just changed rear seal, it was the original rope type. The rear main bearing looked good to me. I checked with plasti gauge. Well within specs, but I’m far from any builder.

                      Ya know I had a weird reading on a cylinder, I thought it was just me. Went right up to 170, then on recheck it only went up to lower 150’s. I’ll have to look it was for sure on odd number drivers side.

                      Edit: ok I looked, can’t tell looking at my chicken scratch writing. Pretty sure it wasn’t 1 & 7
                       
                      Last edited: Feb 7, 2020
                    • skidmark

                      skidmark Well-Known Member

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                      Pictures of cap and rotor. Found rotor tip loose, able to go up about 1/16”. Rivet pulled out pretty easy with pocket knife. Don’t remember part number, it’s the extended tip from Napa. Street fire plug wires, checked #3 wire, exterior looks good, resistance is good. I’ve recently read not to run consecutive firing cylinder wires. I have 5 & 7 run side by side, will fix that.
                      Started leak down testing.

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                      8060B6E0-EB20-4B18-8DAF-7978D770740E.jpeg

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                      • TT5.9mag

                        TT5.9mag Two atmospheres are better than one

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                        Good find on the rotor.
                         
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                        • TT5.9mag

                          TT5.9mag Two atmospheres are better than one

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                          Our of curiosity, what cylinder does the burnt pin in the cap correspond with?
                           
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                          • skidmark

                            skidmark Well-Known Member

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                            It’s #3
                            534C9DCE-FA43-41B3-A81A-E2235A8E46A5.jpeg
                             
                          • TT5.9mag

                            TT5.9mag Two atmospheres are better than one

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                            Kinda figured. Something is causing resistance on that cylinder. Bad plug, plug wire something. Looks like you found your misfire.
                             
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