Acting like one cylinder firing lean.

Discussion in 'Mopar Performance Issues' started by N1ce2k1llu, Jun 23, 2018.

  1. N1ce2k1llu

    N1ce2k1llu Active Member

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    Rebuild 1971 small.block 318LA with all new: Edelbrock 1406, Edelbrock rpm intake, new Promaxx aluminum heads & Lunati Voodoo hydraulic roller cam. Msd 6a, Msd distributor, Blaster 2 coil, msd spark plugs / wires.
    Issue is engine has a misfire or like a lean burn on one cylinder. Kinda goes away at idle when I adjust air/fuel screw a half turn richer on the passenger side. The other 2 cylinders run richer so that is just not a fix. Check ohms on coil & plug wires with meter both good. Can lightly feel the misfire at cruising speeds. Changed out carb springs from the lowest blue up to pink. Im in the idle circuit because mixer screws adjust. This is the 4th intake gasket replacement due too other issues but always been like that. Any ideas?
     
  2. Biff

    Biff Well-Known Member

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    I had a coustomer with an eddy carb surge/ miss changed out the metering rods that fixed it there is a chart with similar symptoms somewhere can’t find it but it’s out there somewhere,
     
  3. 512Stroker

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

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    Nasty problem, what were the other issues? We need the whole story.
     
  4. N1ce2k1llu

    N1ce2k1llu Active Member

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    I have already changed rods for a step richer. Speaking of the carb, drove it last night wot and my idle was 1200rpm in park. The secondaries were cracked open a hair but could manually push the linkage back to shut them. Should the secondarys be cracked open normally?
     
  5. N1ce2k1llu

    N1ce2k1llu Active Member

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    Other issues were from a thumpr cam that I first installed. It shaked like crazy so alot of it was replacing manifold gaskets incase their was a vacuum leak on the underside of the intake. Then I installed a new edelbrock rpm manifold. And the last was replacing the thumpr with a Voodoo cam. The voodoo cam is a big improvement except for the current problem with the misfire or lean spot at idle.
     
  6. 512Stroker

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

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    Every thing you speak of points to a vacuum leak, hi rpm @idle, lean - sorry. Have you measured vacuum at idle? are the secondary butterflies closed? What do the spark plugs look like?
     
  7. mderoy340

    mderoy340 Well-Known Member

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    You need to adjust/center the secondary butterflies so they don't hang up when closing. Idle mixture screws do not have to be the same. Every dual plane engine I've owned needed split jetting to even the burn.
     
  8. N1ce2k1llu

    N1ce2k1llu Active Member

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    Thanks that totaly makes sense now wit a dual plane manifold. So I take it the secondarys should be completely closed at idle? Been looking for info this morning about it, but no luck. Thanks again
     
  9. inertia

    inertia Well-Known Member

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    On pass side of carb , there's a lever on the very outside of the linkages connecting the front to rear shafts, there's a piece that is supposed to push the secondary closed. You likely need to adjust ( bend) the tab to get the sec to close at idle.. Look closely, to understand before you adjust. cheers
     
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    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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      With an Eddie carb, yes the secondaries should be completely closed but not sticking closed. Depending on the cam, the engine often wants extra idle air. If you can make it idle so that the mixture screws are sensitive, with the secondaries fully closed,then you possibly don't have enough idle timing.Make sure the secondaries are not hanging up for other mechanical reasons, like a displaced or wrong gasket,etc.
      If your brake booster supply hose runs off the same leg as the problem cylinder, well you better look into that. My intake did not like that plumbing. I replumbed the booster hose to the back of the carb, but I had A Holley DP. That cured it. You may need a new checkvalve on the booster.
       
      Last edited: Jun 24, 2018
    • N1ce2k1llu

      N1ce2k1llu Active Member

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      Found and adjusted the tab for the secondarys. There was a gap between the two. I don't have a brake booster and currently have my timing @ 18 initial & 18 mech. No vacuum advance. Another reason the shakes bug me is that my cheap summit headers hit my torsion bar and steering box during a shake, should of spent the extra $. About to go outside and re adjust mixture screws now knowing that the screws don't have to be the same amount of turns with a dual intake. Will update results.
       
    • 512Stroker

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

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      Guess what, I spent the money for TTI headers and they hit the torsion bar on the drivers side. Use a vacuum gauge to set your idle screws.
       
    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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      That there no VA might be part of your problem
      Under part throttle operation there are times your engine will want in excess of 50 degrees of timing. Some engines like over 60*. Put a scanner on your OBD car and take it for a ride graphing the advance.
       
    • BigBlockMopar

      BigBlockMopar BigBlockMember

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      Do a compression test to see if one cylinder is lacking.
      Also you could point a lasertemp gun at the same spot on the header tubes on each cylinder to find out which one is running hotter/leaner.

      How/where is your pcv hooked up?
       
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      • N1ce2k1llu

        N1ce2k1llu Active Member

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        This is a 1971 plymouth, no computer to help me out.
         
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        • AJ/FormS

          AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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          No I mean put a scanner on your OBD EFI daily driver, and watch what it does.
          Most Mopar streeters benefit from a well tuned VA system. About the biggest can you can find or build is 22*. So by the time the the rpm rises to stall speed around 1850 on a stocker, he mechanical timing could be 20/25 degrees and so the VA will add it's load of say 20 to that, and the engine will be very happy with 40/45. But as soon as you increase the load by stepping on the gas, the VA starts dropping timing, and by WOT or sooner it has all dropped out. The VA is an automatic timing control, and once tuned you never have to think about it again. The extra advance it brings to the party, allows you to really lean the low speed circuit out, saving you many many gallons of gas over the lifetime of the engine. They say lean is mean, but the only time I agree with that is at part throttle. Your engine will really get snappy, and happy, with a well worked out VA and lean AFR combo.
          I agree with BigBlock as to the use of an IR gun; it will tell you a lot. Hunt for the hottest spot in a straight section of pipe within about 2 inches of the head; corners tend to run hotter. A normal idle temp might be around 350/450
           
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          • 512Stroker

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

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            AJ I agree with you that a well tuned vacuum advance can be benifical, if you have vacuum at idle. Lumpy cams produce little or no vacuum at idle. I have had several hi performance engines that I ran no VA because the engine had little or no vacuum.
             
          • AJ/FormS

            AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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            I agree with you, hence the qualifier "most"
            But you are missing 90% of the story. By 1850rpm the vacuum in the manifold is building pretty strong and by 2200 it has more or less peaked, and holding pretty steady. If you are running 3.55s or better, your Vcan will work exactly as it should, but even better cuz without it your AFR is gonna have to be rich to prevent tip-in hesitations. Sure you can futz with the pump and make it work that way, but it will never run like it would with proper VA assisted timing. When you feel that badboy start pulling on the can and everything smooths right out and the pipes start to sing, and you are just sipping gas, that is the pay off. We are long past lumpy idle.
            I ran a 22*VA even with my 292/108 cam at 11.3 Scr, and reaped the benefits. If you run more than that with an SBM, firstly I can't speak to it, but secondly, a 292/108 is a terrible SBM street cam,lol...... IMO
            Here's what else you can do, and I have done, and continue to do;
            My power timing is delayed to not be all-in until 3200/3400. I have 28* at 2800. I augment the timing with 22* in the can so am cruising at 50* at 2800. So I always have plenty of timing for part-throttle work. With a small 230 cam, and 180psi cylinder pressure, I have a ton of torque available. When I stand on it, the tires light up anyway below 50 mph so I don't care about a few missing degrees from 2800 to 3400.
            So what's the big deal, you ask? The big deal is I burn 87E10 100% of the time, saving about 12cents a mile in fuel costs, and have done so since 1999, over 100,000 miles. And the engine still goes 93mph in the 1/8th, at 3467 pounds. Is $12,000 saved a big deal? I think so.
             
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            • N1ce2k1llu

              N1ce2k1llu Active Member

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              Well, what I have now is a new MSD pro billet distributor with only mechanical. I bought some stoppers 10* & 14* so the highest initial would be 26*+10*=36*. From what I've read is that VA is not necessary. But I know what you guus are talking about, manifold vacuum gives more initial.
              So tried messing with mixture again and the only way I could get rid of the random shake was to unplug my full manifold vacuum port. Drove it but when I came to a red light it would shake a little but go away. Same when slowing down to turn I can hear the header pipes smacking the body. I don't know anymore been troubleshooting it every weekend for 9 months now and I'm over it. Give it a couple more weekends, then to a classic auto shop.
               
            • N1ce2k1llu

              N1ce2k1llu Active Member

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              Put a temp gun on it but pointend it right where the first bend coming off the heads. Just read I should check at a straight line of pipe. also did it when car was off which I know isn't correct but I had an average of 145* on all but one at 115*. Leaner burns hotter so what's up with that? Lol
              Will do a correct reading sometime this week after work.
               
            • N1ce2k1llu

              N1ce2k1llu Active Member

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              Found the missing cylinder #7. Found white corrosion on the plug wire connector at the cap end. Took the cap off to inspect and found corrosion on the rotor contact. Cleaned them up and started the engine. Random shake still there. Was playing with idle screw and when it idle from 800 to 900 rpm it violently shakes alot. I just ordered a Street Demon carb and plan on swapping it with the edelbrock 1406. Hopefully it solves it.
               
            • AJ/FormS

              AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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              Hyup, engine off doesn't count, but fyi;
              With incomplete combustion in the chamber, the charge can continue burning in the pipes.In my experience tho,the temps will be a lot higher, easily double and perhaps triple your readings.
               
            • Abodybomber

              Abodybomber Breaking street machines , since 1983.....:)

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              Power brakes ?
               
            • N1ce2k1llu

              N1ce2k1llu Active Member

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              No power brakes. I tried spraying wd40 around carb and intake for vacuum leaks, no change.
               
            • Abodybomber

              Abodybomber Breaking street machines , since 1983.....:)

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              Seen it ,happen ..good job.
              Feed it ,the fuel.... It 's not fuel injection .