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.
     
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    • 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.
       
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      • 68gtxman

        68gtxman I used to reMember

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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

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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.
         
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        • 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?
           
        • joes68340s

          joes68340s Well-Known Member

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          Well pulled the car out haven’t touched it in 6 months. Still diesels on shut dow. Holley hp 750 dbl pumper. Yes it will run with mixture screws turned in. I have set the initial timing at 10*,14*,20*. Idles 750 rpm. I had issue with pvc so temp removed. Rear throttle blade shut. It pulls about 9 on vac at idle.
           
        • MOPAROFFICIAL

          MOPAROFFICIAL Well-Known Member

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          Run cheveron 91 or better octane then try it...still does it..add a half bottle of stp octane boost...drive around the block and see.
          Mine would do that if I didnt run Chevron premium or use octane boost.
           
        • toolmanmike

          toolmanmike FABO Staff Staff Member FABO Gold Member

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          If it runs with the mixture screws turned in it's bleeding fuel over somewhere. Power valve blown? Floats set too high? Just thinking out loud here.
           
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          • MOPAROFFICIAL

            MOPAROFFICIAL Well-Known Member

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            Missed that, yeah he's running on the mains and not the idle circuit. That can cause issues..
             
          • AJ/FormS

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

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            I've never seen a case of deiseling that wasn't related to a combination of open throttles/ too much air, and something hot in the chambers. But this does not mean colder plugs are the answer. Heat range is chosen by the power run, and perhaps tempered by the primary useage.Changing the idle-timing should very dramatically change the idle speed.
            The principal thing is to choke the air off at shutdown. I have run up to the 292/292/108 DC cam with no dieseling issues.
            You'll get this figured out real soon, cuz 9" vacuum is IMO borderline too low for 750 rpm, for this cam.
            I suggest a LeakDown test to prove the valves are closing, and a compression test to prove the pressure is at least even.
            I know that engine should be able to idle down to at least 650rpm, with a proper Transferport sync; my 292 cammed 367 went there @5* idle-timing. This was 9 degrees retarded from the 14* is usually ran, and the engine still made enough power to drive around the parking lot, by itself, with no slipping the clutch, but just barely,lol. It took that much retard for the power pulses to calm down enough to where the bucking quit, and the car became driveable down there.
            I used a dash-mounted, dial-back, timing delay box; with a range of 15 degrees; which I split to 9retard/ 6 advance, or whatever I wanted on a particular time.
            Oh yeah, sometimes, the throttle-opening has to be fudged because the fuel pull-over is too hard, aka too low a fuel level. On some carbs you can prove this by gently blowing into the float bowl and seeing the idle rpm rise.
            If you just can't get it, then I suggest checking the cam install.
             
            Last edited: Aug 16, 2019
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            • joes68340s

              joes68340s Well-Known Member

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              Thanks still playing with it.
               
            • toolmanmike

              toolmanmike FABO Staff Staff Member FABO Gold Member

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              You will need to do some homework on the correct spark plug and get the proper ones back in. Too cold of a plug will make the engine run like crap and idle rough and rich. If the cam makes the engine idle rough and you can't get it slow enough, you may need a higher stall torque converter. There are many unanswered questions here that need to get asked and answered to fix your problem.
               
            • fklskv

              fklskv Well-Known Member

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              What spark plugs are you using?
               
            • joes68340s

              joes68340s Well-Known Member

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              Back again, rechecked timing have it at 14 initial. Checked secondary throttle blades and they are closed. Added 3/32 bypass in primary blades. Runs better throttle is closed up more but I can still close the mixture screws it smells better but still diesels. Idles 750 rpm.
               
            • fklskv

              fklskv Well-Known Member

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              The only other thing I can think of where there is no effect turning the mixture screw is the power valve.
               
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