detonation / knock issue on 340

Small Block Mopar Engine

  1. shimsham1

    shimsham1 Well-Known Member

    Messages:
    140
    Likes Received:
    16
    Joined:
    Jun 29, 2009
    Location:
    new york
    Local Time:
    1:00 AM
    I don't know the specs of the engine, I didn't build it and the last owner didn't know.
    curious - what carb would you suggest?
     
  2. MOPAROFFICIAL

    MOPAROFFICIAL FABO Gold Member FABO Gold Member

    Messages:
    11,711
    Likes Received:
    11298
    Joined:
    Jun 1, 2016
    Location:
    Oceanside ca.
    Local Time:
    10:00 PM
    You more than likely already diagnosed it in its entirety. The carb base gasket and possibly the carb throttle plate to body.
    Start at the base gasket, cheap n easy and you know it leaks by the testing you initially did. Do it before anymore books are thrown at it.. lol then look over the timing and its curve...jetting and power valve if so.
     
    • Like Like x 1
    • cudavert

      cudavert DARTAHOLIC

      Messages:
      2,804
      Likes Received:
      475
      Joined:
      Apr 16, 2009
      Location:
      Hesperia, CA
      Local Time:
      10:00 PM
      Know what you're jetted at too, as was mentioned. Your distributor may have some quicker advance springs in there....find out the curve. Another thing, if this motor burns a bit of oil, that can definitely cause detonation issues.
       
      • Like Like x 1
      • G70464

        G70464 Active Member

        Messages:
        32
        Likes Received:
        33
        Joined:
        Aug 23, 2021
        Location:
        Tampa, FL
        Local Time:
        1:00 AM
        When you get the vacuum leaks solved it needs at least a 700/750 Holly or a 700 demon. I would start with a 800 thermoquad, i like them and can tune them, that is what originally came on a 340.
        But first you really need to find why it has a giant air leak.
        I would also do a compression test to see how hot that motor is. Mine is a 10.5 to 1 and have compression around 200. It just gives an idea of how much squeeze your cylinders are giving. I need 93 and up to run. It is in Charger but B Body sites barley talk about small blocks. 20170415_123842.jpg
         
        • Like Like x 2
        • Babyblue66

          Babyblue66 Cool dude

          Messages:
          759
          Likes Received:
          200
          Joined:
          Oct 29, 2007
          Location:
          In an apartment, MN
          Local Time:
          12:00 AM
          What's total timing and what rpm is it all in by?
           
          • Like Like x 1
          • shimsham1

            shimsham1 Well-Known Member

            Messages:
            140
            Likes Received:
            16
            Joined:
            Jun 29, 2009
            Location:
            new york
            Local Time:
            1:00 AM
            Updates. Found the carb base plate is cracked. So I’m in search for a good replacement. I did do a compression test. Highest cylinder reading 180 lowest 165. The plugs I just put in 30 miles ago look like this. Guess I’ll continue the work after either I find a replacement base plate or a carb. I’ll follow up once I’m back into it

            BA93A83F-CA74-4FC7-B600-7FD0D5CE082C.jpeg

            8989501A-C31D-42CD-8B7E-35974D2E790D.jpeg

            CAECC5BE-2FBA-40C3-A838-8E85C5945BDE.jpeg

            3EC8514C-56F5-468F-8B32-DECB6ED9453E.jpeg
             
            • Like Like x 1
            • Agree Agree x 1
            • RustyRatRod

              RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

              Messages:
              88,448
              Likes Received:
              71744
              Joined:
              Jun 7, 2010
              Location:
              Georgia
              Local Time:
              1:00 AM
              Well.......at least you found the issue. Good. Sorry that's what it is, but I'm glad you found it!
               
              • Agree Agree x 1
              • 512Stroker

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

                Messages:
                3,382
                Likes Received:
                2759
                Joined:
                May 30, 2016
                Location:
                Freedom, MO
                Local Time:
                12:00 AM
                Dead lean, some of which being caused by your busted carb plate. Timing is way shy.
                This may help
                FBO Ignition - Home of the HRR688 - Spark Plug Reading
                 
                • Like Like x 1
                • Agree Agree x 1
                • Babyblue66

                  Babyblue66 Cool dude

                  Messages:
                  759
                  Likes Received:
                  200
                  Joined:
                  Oct 29, 2007
                  Location:
                  In an apartment, MN
                  Local Time:
                  12:00 AM
                  180psi cranking is pushing it with pump gas and an open chamber head.
                   
                  • Agree Agree x 4
                  • mgoblue9798

                    mgoblue9798 Well-Known Member

                    Messages:
                    370
                    Likes Received:
                    252
                    Joined:
                    Mar 16, 2015
                    Location:
                    atlanta
                    Local Time:
                    12:00 AM
                    Depends on the altitude and octane but you are right for sea level pump swill. 340 I built in my Challenger many moons ago would run ok on 93 during the winter in Atlanta, but as soon as it got hot during the summer it would run on and knock like hell. Had to run octane boost. I also knew nothing about rounding the sharp edges and cleaning up the chambers back then so it likely would have made enough difference if I had.
                     
                    Last edited: Apr 30, 2022
                    • Like Like x 2
                    • Babyblue66

                      Babyblue66 Cool dude

                      Messages:
                      759
                      Likes Received:
                      200
                      Joined:
                      Oct 29, 2007
                      Location:
                      In an apartment, MN
                      Local Time:
                      12:00 AM
                      I fix this problem in my friends GTO by putting in a cam with more duration on a wider lobe separation angle to close the intake valve later. Is 10.2 to 1 but it has quench, and it runs great on 91 now. Changing a cam can make an engine that would otherwise not run on Pump Fuel, work okay
                       
                      • Agree Agree x 2
                      • mgoblue9798

                        mgoblue9798 Well-Known Member

                        Messages:
                        370
                        Likes Received:
                        252
                        Joined:
                        Mar 16, 2015
                        Location:
                        atlanta
                        Local Time:
                        12:00 AM
                        Be sure to hang on to that paper with the cylinder pressure readings.

                        If all else fails and you cant get a tune without detonation, you may have to pull the heads to check for hot spots or carbon build up. If so, the highest pressure cylinder chamber would be the one I would look at first. Polishing the valve faces, smoothing the chamber, and knocking down sharp edges on the head would likely get you back to good if needed.
                         
                      • RustyRatRod

                        RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

                        Messages:
                        88,448
                        Likes Received:
                        71744
                        Joined:
                        Jun 7, 2010
                        Location:
                        Georgia
                        Local Time:
                        1:00 AM
                        Even with all the A/C gobbledygook half coverin the engine up, that's a good lookin engine!
                         
                      • G70464

                        G70464 Active Member

                        Messages:
                        32
                        Likes Received:
                        33
                        Joined:
                        Aug 23, 2021
                        Location:
                        Tampa, FL
                        Local Time:
                        1:00 AM
                        I live in Florida... Air is a necessity.
                        Thanks. It runs as good as it looks.
                         
                        • Agree Agree x 1
                        • G70464

                          G70464 Active Member

                          Messages:
                          32
                          Likes Received:
                          33
                          Joined:
                          Aug 23, 2021
                          Location:
                          Tampa, FL
                          Local Time:
                          1:00 AM
                          Wow that is scary lean. Thankfully only 30 miles.
                          See how it runs with a new carb, try to get something in the 700 cfm range or an 800 thermoquad.
                          I am not sure you can run 91, see if it is OK after new carb. I run 93 non ethanol in mine timing 34° all in at 2800.
                           
                        • RustyRatRod

                          RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

                          Messages:
                          88,448
                          Likes Received:
                          71744
                          Joined:
                          Jun 7, 2010
                          Location:
                          Georgia
                          Local Time:
                          1:00 AM
                          It's CLEAN as can be! I give you that. I'm in middle Georgia. I just suck it up. lol
                           
                          • Like Like x 1
                          • Thanks! Thanks! x 1
                          • Hysteric

                            Hysteric Well-Known Member

                            Messages:
                            1,874
                            Likes Received:
                            1446
                            Joined:
                            Dec 31, 2005
                            Local Time:
                            12:00 AM
                            People have run much more than that and there are other factors involved than just cylinder pressure.

                            240 psi on pump is possible when you know what you're doing.
                             
                            Last edited: Apr 30, 2022
                          • Hysteric

                            Hysteric Well-Known Member

                            Messages:
                            1,874
                            Likes Received:
                            1446
                            Joined:
                            Dec 31, 2005
                            Local Time:
                            12:00 AM
                            Is that yellow or tan on the strap?

                            Next time you take photos of plugs wrap them in your hand and take a close up so we can see what it looks like into the shell.
                             
                            • Like Like x 1
                            • Agree Agree x 1
                            • MOPAROFFICIAL

                              MOPAROFFICIAL FABO Gold Member FABO Gold Member

                              Messages:
                              11,711
                              Likes Received:
                              11298
                              Joined:
                              Jun 1, 2016
                              Location:
                              Oceanside ca.
                              Local Time:
                              10:00 PM
                              180psi is edgy but will run good with the correct tune.
                               
                              • Agree Agree x 1
                              • Rat Bastid

                                Rat Bastid Dunamis Metron

                                Messages:
                                2,038
                                Likes Received:
                                1928
                                Joined:
                                Nov 23, 2020
                                Location:
                                22 The Avenue
                                Local Time:
                                10:00 PM
                                It’s not lean.
                                 
                                • Disagree Disagree x 1
                                • fishmens67

                                  fishmens67 FABO Gold Member FABO Gold Member

                                  Messages:
                                  13,167
                                  Likes Received:
                                  14664
                                  Joined:
                                  Nov 10, 2012
                                  Location:
                                  idaho
                                  Local Time:
                                  12:00 AM
                                  It can still detonate in the higher RPM range as cylinder pressures increase.
                                  It's harder to hear and diagnose.
                                   
                                  • Agree Agree x 2
                                  • Rat Bastid

                                    Rat Bastid Dunamis Metron

                                    Messages:
                                    2,038
                                    Likes Received:
                                    1928
                                    Joined:
                                    Nov 23, 2020
                                    Location:
                                    22 The Avenue
                                    Local Time:
                                    10:00 PM

                                    What carb is it? If it’s a Holley call BLP or maybe ProForm is still making parts and get a billet plate.
                                     
                                  • fishmens67

                                    fishmens67 FABO Gold Member FABO Gold Member

                                    Messages:
                                    13,167
                                    Likes Received:
                                    14664
                                    Joined:
                                    Nov 10, 2012
                                    Location:
                                    idaho
                                    Local Time:
                                    12:00 AM
                                    It would take variable cam timing to run 240 psi.
                                     
                                  • Hysteric

                                    Hysteric Well-Known Member

                                    Messages:
                                    1,874
                                    Likes Received:
                                    1446
                                    Joined:
                                    Dec 31, 2005
                                    Local Time:
                                    12:00 AM
                                    Nope.
                                     
                                  • Rat Bastid

                                    Rat Bastid Dunamis Metron

                                    Messages:
                                    2,038
                                    Likes Received:
                                    1928
                                    Joined:
                                    Nov 23, 2020
                                    Location:
                                    22 The Avenue
                                    Local Time:
                                    10:00 PM

                                    You can do it. You just have to have a purpose going in.

                                    And that’s one reason why cranking compression isn’t a very good predictor of detonation.
                                     
                                    • Like Like x 1
                                    • Disagree Disagree x 1
                                    1. This site uses cookies to help personalize content, tailor your experience and to keep you logged in if you register.
                                      By continuing to use this site, you are consenting to our use of cookies.