help fuel or ignition

Discussion in 'Small Block Mopar Engine' started by dust buster, Aug 11, 2018 at 11:36 AM.

  1. dust buster

    dust buster dust buster

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    Hi all, I have a 1975 Duster 360, just built a new motor, it is a 1994 360 magnum, now a 408 stroker new eq heads, hughes cam around 530 lift 9.5 cr, 750 qf slayer carb. Problem when warm 190 degree does not want to idle and breaks up at 4500 and up, tried every adjustment known to man cannot get it right any thoughts?
     
  2. 67Dart273

    67Dart273 Well-Known Member

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    Too lean or something dropping ignition power/ voltage to the plugs. Could be a lot of things. You have a wideband O2? At the price of those things I'd have "another." I would have "killed" to have had one when I was a young'in

    Detail your ignition system..............and start by measuring the voltage supply to it
     
  3. Murray

    Murray FABO Gold Member FABO Gold Member

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    So it idles OK when it is cold but not when it is warm? So is this a carb that is coming off the fast idle cam? Poor curb idle speed or A/F ratio? Timing, coming off the vacuum advance?
     
  4. AJ/FormS

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

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    That was my thought too. Put the Vcan on the sparkport and retime it to 14*, let's see what that does.
    You do have a working PCV system..... right
     
  5. nm9stheham

    nm9stheham Well-Known Member

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    And if you do have a PCV, what model/type of PCV is it? Your cam is likely going to make low vacuum, and any stock type PCV at low vacuum is going to be opened up to cruise flow, which is too open for idle conditions. Also, get a vacuum gauge and measure vacuum in the intake (under the carb) at idle and report that, please.

    Work on the idle issues first, and then address the high RPM breakup. They may well be 2 different issues.
     
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    • fez440

      fez440 Well-Known Member

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      Just curious if he has pcv vavle issue what do you guys use on engine with low Idle vacuum ?
       
    • trapster

      trapster Well-Known Member

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      Sounds like a weak coil, or low voltage on coil.
       
    • mycuda

      mycuda FABO Gold Member FABO Gold Member

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      Seems lean after engine to temp.
      try retarding abit.

      cam duration?
       
    • nm9stheham

      nm9stheham Well-Known Member

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      I've used a Fram 191; that has a different size so can't go in the stock valve valve cover grommet. But there is a Mopar replacement part... I just can't find the number. If you search about PCV's here on this FABO SBM subforum, you'll find it listed a coupla time within the last 2 years.

      The alternative is an aftermarket adjustable one that costs over $100.
       
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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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        On a really big cam IDK.
        The biggest I have tuned have been the 340/360s with the 292/508's,which were still tuneable with a single PCV, operating as an Idle Air bypass,and using a non-4-corner idle, carb. The Bypass makes it easier to set the T-port sync. Without it, you have to figure out an alternative way to give the engine the idle air she craves. Some guys crack the secondaries, and some guys drill small holes right thru the primary throttle valves, which I prefer.
        Don't worry, as soon as you crack the throttle, the manifold vacuum will rise and the PCV will operate normally, helping to keep your engine clean inside..
         
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