380 hp Magnum crate dieseling problem

Mopar Performance Issues

  1. joes68340s

    joes68340s Well-Known Member

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    I have a 360/380hp crate with a 4spd coldest plugs I can find idle set at 750 timing 32 and 180 stat. It also has a Pertronic in the distributor. It diesels at every shut down when warm.
     
  2. Murray

    Murray FABO Gold Member FABO Gold Member

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    Why do you need colder plugs?
     
  3. fklskv

    fklskv Well-Known Member

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    How much are your throttle plates open?

    My guess is more timing and closing throttle plates (idle adjustment) more.
     
    • Agree Agree x 4
    • joes68340s

      joes68340s Well-Known Member

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      Yes I am going to try that again.
       
    • Vamisk

      Vamisk Well-Known Member

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      What carb? Will it idle if you close the idle screws?
       
    • joes68340s

      joes68340s Well-Known Member

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      750 double pumper . I know I have to leave the screws turned in less than full turn. I will re check carb.
       
    • 68gtxman

      68gtxman I used to reMember FABO Gold Member

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      If all else fails, shut it down with the clutch pushed in and the trans in gear. Turn the key off and let out the clutch (holding the brake too). My 225 did this in 1975 and the only way to stop it was turn off the key while the trans was in gear.
       
    • AJ/FormS

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

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      That crate motor has the 288/292/108 cam in it right? It likes a lotta idle air bypass. You are probably up the transfers too far, and when you shut her off the flywheel momentum continues to spin the crank. This pulls in air and fuel past the too far open throttle blades, and that mixture is exactly the same as it was before you killed the ignition. So just the heat of compression can cause the mixture to ignite.
      The cure in my case was to close the secondaries up tight but not sticking, then set the T-port sync to a little taller than wide, open up the mixture screws to 3/4 turn,make sure the PCV is working,(you are running a PCV right?) and the Vcan is on the sparkport, and I set my timing to 14* with a 4 speed, otherwise that cam gets jumpy with more. Then you have to give it bypass air somewhere, else the idle speed will be too slow, and stalling on take-off becomes an issue. My solution was to drill holes in the primary throttle blades, on the Transfer port side about half way between the shaft and the edge. The tricky part is to choose the right hole-size. So I started small, and snuck up on it. If you go too big don't panic, you can solder up the hole, move over, and start again.
      Start with one hole in each primary blade at 1/16. I know 1/8th in each will be too big. I think I ended at 3/32s; your results may vary.
      If this scares you; you can Tee into the PCV line with one of those 5port tees, and use Holley jets in small hoses, to determine what your engine likes. If you spend a lotta time at below 2000 rpm, and you find it jumpy down closer to idle, you can soften the pressure pulses by running less timing; I run 14* with a 3.09x3.55=10.97 starter gear. It will idle down to 550 in low-gear at 5* advance. If you have been running a Vcan on manifold vacuum, you will have to give that up; put the can back on the sparkport.
      Then go back and fix your power timing, and rate of advance, and tune in the Vcan as early, and as fast, and as much, as you can get, I run 22* in that can.
       
      Last edited: Oct 1, 2018
    • moper

      moper FABO Gold Member FABO Gold Member

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      Check to make sure the secondaries are not open a hair. There's a recessed screw you can reach with the carb off the engine. I've seen some that came to me after someone had cracked the secondaries open to help idle rather than fix the problem. There's no reason that crate engine should need a ton of timing to idle.
       
      • Thanks! Thanks! x 1
      • joes68340s

        joes68340s Well-Known Member

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        Thanks for replies I need to go through everything again.
         
      • MOPAROFFICIAL

        MOPAROFFICIAL Well-Known Member

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        Like said, check for the throttle blades open too much ="idling on the main circuit " which usually just makes for stink rich idle...but I guess it can also cause run on, but mostly coupled with extra air leaking in..Vacuum leak. As you stated already...the idle mixture screws , double pump as stated so we know there are 4, are only a 1/2-3/4 out...so let's explore the next thing.

        If the cylinder psi is in question and carburetor checked to be correctly adjusted..you would approach it by taking some timing out ... or running premium like those motors require....and octane boost ,sometimes.


        What is the initial timing/at idle?
         
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