Think I May Have Found Out Why My 340 Seemed to Idle Rough...

Mopar Performance Issues

  1. Divenut

    Divenut Well-Known Member

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    Hey Folks,

    My 340 has forged crank/rods internally balanced, and has idled rough while idling in gear since the day I bought it. Not just a cam with lots of overlap rough, but like a non-firing cylinder type of rough. Did all the basic troubleshooting. Compression test, wires, checked for vacuum leak, etc. all cylinders were firing. But it ran well overall. Weird...?

    As some of you may know, the Duster's been out of action while I perform some much needed (and long put off)repairs. One of which being replacing the seals on the 727, i.e. front pump seal, bushing, shifter pivot seals, replace the converter etc. The car is a 4 speed car that someone swapped a 727 into. While I had the trans out I researched the part number and looks like it is from a 72 360 vehicle. So, the new converter arrived yesterday and I compare it to the one that was in the car. When I notice a difference. The old one has weights welded onto it....the new one for the internal balanced 340 does not.

    Am I wrong or does that old TC look like one for a externally balanced 360? And would that cause that untraceable rough idling in gear?

    Be well
    Pat

    20170823_180857_resized.jpg Resized_20170618_115758.jpg
     
  2. TrailBeast

    TrailBeast Slightly Twisted Member

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    The bottom one is externally balanced, so if you have an internal balance on the motor and a neutral flex plate you are going to have a vibration.
     
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    • Dartnut

      Dartnut Don't hate me because i'm beautiful

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      In my opinion, it would have caused the engine to vibrate at all speeds, not just idle.
      Bolt the new one onto the factory flex plate or a new one designed for neutral balance and it will be a lot better.
       
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      • LeeinRI

        LeeinRI Active Member

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        I don't believe those weights are for external balance. I am pretty sure they are there to balance the converter assembly. If it were an externally balanced converter the weights would be on the face of the converter where the flexplate bolts on. The four mounting lugs (one has an offset hole) are used as a refference to position the weights so when installed in relation to the crankshaft they are in the correct position. Plus, that appears to be a high stall aftermarket converter, I have never seen a performance aftermarket converter that is externally (Detroit) balanced. I have however seen a 340 that was rebuilt where someone installed a 273/318 forged crank. That thing vibrated like a washing machine, more so at idle. The 340 forged cranks have holes drilled in the first and last rod throws to lighten them due to 340's using heavier rods.
         
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        • Divenut

          Divenut Well-Known Member

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          In hindsight, I'm sure that there was in fact some vibrations throughout the range Dartnut. But is was just so damn noticeable in gear at idle that the higher rpms seemed "better"!

          Pat
           
        • Dartnut

          Dartnut Don't hate me because i'm beautiful

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          I've seen them vibrate like a 50 year old Maytag!
           
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          • LeeinRI

            LeeinRI Active Member

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            It will quickly destroy the main bearings also! I would strongly recommend pulling the pan and physically checking everything to be sure! I once had a 67' 273 four barrel engine that I bought cheap. I soon found out why, it vibrated but only at 3K plus RPM! I pulled the pan and found a sleeve in #2 cylinder and a 340 rod installed in that location. Lmao! I changed the rod and all was well. For some reason that particular 273 was very powerful compared to others I have had.
             
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            • Divenut

              Divenut Well-Known Member

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              Thanks Lee,

              The pan, windage tray, pump are off since I also replaced the leaking rear main seal while the tranny's out... Everything looks to be good. I'm sure...I think! LOL

              Pat
               
            • airgrabber

              airgrabber Well-Known Member

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              Cast crank 340 motors got weights welded onto the converters, from the factory.
               
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              • daddio4048

                daddio4048 Daddios72

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                I made the same mistake. Shook like a washing machine. When they are suppose to be there on a external balance or not on a internal balance engine; it shows up big time ! Nothing beats having a professional balanced engine !
                 
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                • Divenut

                  Divenut Well-Known Member

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                  Ya, it's weird, the motor clearly was built with forged crank/rods and was balanced when built. Guessing the swapped trans & converter were from a 360 donor... gotta love what one finds when acquiring other folks project car.

                  Looking forward to getting it back together
                  Pat
                   
                • daddio4048

                  daddio4048 Daddios72

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                  Will be like a night & day difference !
                   
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                  • d55dave

                    d55dave Well-Known Member

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                    Those are converter balance weights. Most aftermarket converters are balanced, much like your tires are balanced. Those are not engine balance weights. Engine balance weights are much larger and are on the forward face of the converter. I bet that if you take that Hughes converter out of the box and look at it you will also see weights welded on it at some point. If not I would be calling hughes to ask if they have balanced the converter.

                    I strongly doubt this is the cause of your vibration. Does the car have solid engine mounts, is the engine chained down?
                     
                    Last edited: Aug 25, 2017
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                    • toolmanmike

                      toolmanmike FABO Staff Staff Member FABO Gold Member

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                      that may take out the front tranny seal and may be damage the bushing.
                       
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                      • toolmanmike

                        toolmanmike FABO Staff Staff Member FABO Gold Member

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                        There isn't much in a torque converter that can be out of balance that far. (that far)
                         
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                        • d55dave

                          d55dave Well-Known Member

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                          Divenut, pm me if you are interested in selling the grey converter.

                          Cheers,
                           
                        • Divenut

                          Divenut Well-Known Member

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                          Sorry for the delayed response Dave. PM sent
                           
                        • Dartish

                          Dartish Well-Known Member

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                          360 converter has big weight welded to the front facing the engine. Those weights welded around the sides have nothing to do with engine balance
                           
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                          • moper

                            moper FABO Gold Member FABO Gold Member

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                            There are two possibilities: The weights on the side are only for the convertor's balance... But they are huge if they're for that. Normally it's maybe a chunk of 1/16" thick steel. So it's either weights from a shop's lower end balance job or the 360 weight that normally goes on the flexplate side of the convertor was put on the side (not cool).
                             
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                            • Divenut

                              Divenut Well-Known Member

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                              Yes, that's what I found odd Moper. While I have seen small balancing tabs on converters, I've never seen the number and size of weights that the old TC has on it. Guess we'll know once I get the Duster back together. I'll be sure to post the follow-up results here .....good or bad.

                              Be well
                              Pat
                               
                            • dartman1965

                              dartman1965 Well-Known Member

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                              Sure you have the correct balancer..... ?
                               
                            • Divenut

                              Divenut Well-Known Member

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                              Hey Dartman,

                              Yep. Have the balancer for the internally balanced 340. :) Thanks for the suggestion man.

                              Pat
                               
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