Rough idle issue

Slant 6 Engines

  1. briwill70

    briwill70 Well-Known Member

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    Trying to diagnose rough idle on new engine. New HEI ignition, tested wires, NGK ZFR5N plugs, Weber 38 adjusted to best lean idle and timing at initial +15 degrees. Also snugged down all intake bolts and does not appear to have any leaks. Runs and accelerates great but stumbles a bit when idling. Really bad in gear.

    Has a Oregon 2106 cam, adjusted lash to .015 IN/EX per cam card while on the stand. Maybe too tight? Other thoughts?
     
  2. dano

    dano Evil Handy Man

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    Too lean at idle could be your issue. Have you tried adding a little fuel to see if it changes anything? Do you have a wide band to measure the o2?

    Today's fuel lights off way fast and is designed for fuel injection, so in this heat (cooler than it has been) you might be getting some vapor lock for bowl boil.

    Other thoughts, do you have vacuum advance hooked up? If so is it to a ported vacuum source or full vacuum? It should be connected to ported. What is the mechanical advance in the distributor?
     
    Last edited: Aug 4, 2020
  3. briwill70

    briwill70 Well-Known Member

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    I don't have a O2 sensor but I don't think fuel is the issue. Sounds like a miss to me. Dist hooked to ported vacuum and is a new old stock electronic unit. Will probably check plugs, do compression check and adjust valves next to rule out any mechanical issues.
     
  4. Rmoore

    Rmoore Well-Known Member

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    Think I would take some brake clean and spray all the vacuum hoses and fittings !! Base of carb or weber what ever you have , and intake manifold !! You will find your vacuum leak if you have one !
     
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    • AJ/FormS

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

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      Isolate the cylinder
       
    • teringer

      teringer ase master hall of fame FABO Gold Member

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      put a shop towel ( cloth) on top of carb and see if it idles better if it does its lean
       
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      • j par

        j par Well-hung Member

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        When you say you did this "on stand"?..
        Was this on a engine stand or an engine run stand? As in did you do it while the engine was hot? Because that's what your cam card is calling for when the engine is hot and yes it changes...
        What does an Oregon 2016 cam specs consist of? So I don't have to go find it I assume this is a slant six?...
         
      • slantsixdan

        slantsixdan =..=

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        The Oregon 2106 cam is the Dutra RV10 (vs. the 2106R, which is the RV10-RDP). It's an optimised version of the MP 243 cam (which was 244°/244°/28°/0.436"). Nowhere near a big enough cam to cause rough idle; there's something the matter here and I'm guessing it'll be found in the carburetion department, but you might try backing off your valve adjustments to 0.020"/0.020" (hot and running per valve adjustment procedure) and see if it does anything for your rough idle. If not, it's easy enough to change back.

        15° BTDC is a lot of initial timing—could easily be too much. Some time and effort or money invested in dialling-in your advance curves so everything's happy at 5° to 7° BTDC will pay back handsomely. Talk to DusterIdiot over on slantsix.org if you're not up for recurving you distributor on your own.
         
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        • briwill70

          briwill70 Well-Known Member

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          Thanks. I drove the car around last night to get it hot and re-adjusted the valves. Was surprised that my cold on engine stand adjustment was pretty much dead-on at .015. I opened it up to .016 and drove again. I can now hear the slightest chatter from the rockers. Set the lean best idle again and it seems to be smoother now. Will drive it for a couple days and maybe open up to .018 to see if that further changes things. I'm open to recurving the distributor. It's a new old stock from OCPNW. Just trying to get the old girl smoothed out first.
           
        • halifaxhops

          halifaxhops It's going to get stupid around here! FABO Gold Member

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          Find some one that still has a scope and can read it it will definitely let you know what the ignition is doing.
           
        • briwill70

          briwill70 Well-Known Member

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          Interesting new development. I saw some moisture around the block off plate on my Super six intake. Removed the block off plate and some fuel dribbled out. Was kinda wondering when I installed if fuel would pool in this area through the hole in the intake. Has anyone had this issue? Also possible this was a vacuum leak but kinda doubt it since it was holding fuel.

          IMG_8737.jpg
           
        • volaredon

          volaredon Well-Known Member

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          I have a super 6 intake that I am getting ready to put it on to my son's truck, and the port that you show, I tapped out under the carb and put in a pipe plug , with red loctite for insurance.
          My 85 came to me with no manifolds under the hood/ but a pre egr intake and exhaust manifold in the bed. Just a 1 BBL for now but good enough to verify that I have a running engine. I have a super 6 setup for it too.
           
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