383 Cruising Issue

Mopar Performance Issues

  1. 6kevin8

    6kevin8 Well-Known Member

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    I'm having an issue when cruising in my dart for somewhat long trips on the freeway. When I go over 20 miles or so, if I try to push the car it'll start to backfire and act like its not getting fuel. It makes this noise like "k-thunk, k-thunk, k-thunk, k-thunk" until I let off the throttle.
    The car has a pretty stock 383 with schumacher headers. The 727 has a 2200 stall. I was in an accident recently so I don't have a full exhaust system... it's cut off right after the mufflers. I don't remember having this issue prior to my accident, but there are some changes that the car has had since. The rear end now has 3.23s and a true trac instead of 2.94s. I'm also running 27" tall wheels in the back now. When I'm on the freeway, I'm usually cruising at 4000 rpm give or take. Also, my car before didn't have a hoodscoop. Quite a few new variables, but any thoughts?

    Dart.jpg
     
  2. AJ/FormS

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

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    The math says; 4000~94 mph with 3.23s and 27s. That sound right?

    My guesses:Overheated sparkplugs due to;
    A)wrong heatrange ...... Move colder 2 heatranges and/or
    B) lean running,..............Fatten up the main jets 2 sizes and make sure the fuel level is and remains correct,and/or
    C) insufficient timing........Make sure the Vcan is functioning,and the total timing is near 55*@4000@cruise.
    D) engine running hot.
    then roadtest.
     
    Last edited: Aug 28, 2018
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    • Murray

      Murray FABO Gold Member FABO Gold Member

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      Or can the carb be starving? Low float level, clogged fuel filter or gas tank filter?
       
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      • 6kevin8

        6kevin8 Well-Known Member

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        I'm not cruising say 94, I'm thinking the shop who did my rear end might've put 3.55s in it.
         
      • 6kevin8

        6kevin8 Well-Known Member

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        I do wonder about starving. The carb is a 600 cfm Holley but I mean, these things used to come as 2 barrels right?
         
      • 6kevin8

        6kevin8 Well-Known Member

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        We're going to check how accurate the tach is too.
         
      • My70b

        My70b Well-Known Member

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        Happened to my daugthers dart coming back from Carlisle the triangular plug on the orange box kept coming loose fixed that and perfect

        dart1.jpg
         
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        • 6kevin8

          6kevin8 Well-Known Member

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          what triangular plug and orange box?
           
        • AJ/FormS

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

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          Holy smoke that looks very similar to my first Dodge, a 70Swinger. Beautiful car
           
        • AJ/FormS

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

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          That's probably a good idea cuz 3.55s and 27s comes to ~86@4000,
          65 should be about 3000.
           
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          • 6kevin8

            6kevin8 Well-Known Member

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            My dad is the wizard when it comes to these cars, but just to hear opinions... is 4000 rpm too high for cruising when it comes to these engines?
             
          • Alaskan_TA

            Alaskan_TA Well-Known Member

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            The aftermarket fuel pump push rods are notorious for wearing out, so you can check that.
             
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            • AJ/FormS

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

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              I wouldn't cruise there because;
              A) that's a lot of wear and tear on the engine About 70% more.
              B) NVH. Noise,Vibration and Harshness.
              4000 at 65 is friggen loud. And
              C) the driveshaft is whipping around something fierce.
              D) the rear end is running a lot warmer, and is gonna want a better oil.
              E) the automatic is gonna run warmer too..... and it puts it's heat right into the coldest part of the rad...... which then immediately goes into your engine. So your cooling system has to be up to the task.
              F) RPM Sucks gas.
               
              Last edited: Aug 29, 2018
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              • 6kevin8

                6kevin8 Well-Known Member

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                All makes sense to me. I'll just cruise with it at 3000 +/- especially since I don't have a dedicated trans cooler. I also want this engine to last me for a few more years at least lol.
                 
              • Mattax

                Mattax Just the facts, ma'am FABO Gold Member

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                If its backfiring in the exhaust then there is unburned fuel there, plus enough heat to ignite it of course.
                Running out of fuel it sometime coughs and then dies.
                Take out a few spark plugs and see what they have to show.
                 
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                • 6kevin8

                  6kevin8 Well-Known Member

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                  I don't think it was backfiring, and the tank probably had 15 gallons in it when this occurred. The spark plugs are practically new.
                   
                • 7milesout

                  7milesout Well-Known Member

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                  Dredging up an old thread because I found it interested.

                  6kevin8 - One of the first things I did was replace the exhaust when I acquired the Scamp. When I did so, I added a bung to each side's exhaust. Then I put an AFR gauge in (just one, swap from side to side when I need / want to, just to balance or confirm balance). It has been the most satisfying thing I have done to the car in 1.5 years worth of a long list of things I've done to the car (mainly all maintenance and repair after the car sat for 15 years).

                  I tell you this for 2 reasons. 1) The sound you tried to recreate on this thread, the "k-thunk, k-thunk, k-thunk, k-thunk" sounds like what I have grown accustomed to as a lean sound. I would have never guessed that before the AFR gauge. I might be wrong, because of the differences between mine and your interpretations as to what we hear. And, 2) if you have the ability at all, I strongly suggest putting an AFR gauge on it. It will answer so many questions.

                  I wonder if those long drives on the interstate are building up just enough heat to start boiling gas in the lines. But the gas is moving fast enough that most of the time the gas doesn't fully vaporize before it gets to a the carb. Don't know if hills would make it better or worse. I would generate more heat, but it would use fuel more rapidly too. But, some level of vapor lock might be slightly starving the carburetor causing it to go lean. Just a theory.

                  It's been ~4 months, maybe you've figured out what the problem really was. I would be interested to know.


                  7milesout
                   
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                  • rumblefish360

                    rumblefish360 so close yet so far away

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                    Your not getting a lot of help here except from a few. Check your fuel filter for an old clogged up one, check fuel pump pressure after it runs a while or to that point of coughing and hacking. The pump maybe going south when it gets heat soaked from driving long periods, the sock in the gas tank may also be getting clogged.

                    Why AJ is making constant mention of high speed driving rpm is beyond me.
                     
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                    • TrailBeast

                      TrailBeast Slightly Twisted Member

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                      Also make sure your exhaust didn't get bent closed somewhere farther up.
                       
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                      • 7milesout

                        7milesout Well-Known Member

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                        I also meant to add a couple things: Your car looks awesome with those wheels. Helpful or not.

                        I hope my post above was helpful. Another thing besides the k-thunk noise that makes me believe it is / was running lean is that you said, "if I try to push it." In my mind that means you're trying to accelerate to go even faster … which will use more fuel and run the float down more. Yeah, sure it should handle it. But sustained periods of 4,000 rpm uses a lot of fuel. Using a lot of fuel creates heat. Both could lead to just enough vapor lock to not allow the bowl to be full enough.

                        My Scamp turns pretty much 3,000 rpm at 60 mph with 24" tires, 3.23 gears and a TrueTrac. To be accurate, the tachometer shows right at 3,000 rpm. But my tach sort of freezes when I accelerate hard. Cruising along it doesn't freeze, but the freezing tends to make me believe it's not so accurate. So another tach will be sourced next year. RPM tools on the internets says that my combo should be turning 2,713 - 2,786 rpm. But it sounds like its really screaming. I can't even imagine 4,000 rpm.
                         
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                        • rumblefish360

                          rumblefish360 so close yet so far away

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                          He is running 3/23’s and a 27 inch tire.
                          He is t running at 4000 rpm!
                          You read what knuckle head AJ wrote & ran with it.
                          Why the hell AJ went there, which has no bearing on his problem is beyond me.

                          Why everyone else (that did) continue to run with it is a mystery.
                           
                        • 7milesout

                          7milesout Well-Known Member

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                          I'm not trying to make an argument. But I'm going to say that I'm not following your logic. The O.P. said he's turning 4,000 rpm. But even if that's not accurate, it seems that the O.P. is running high rpm. And high rpm would use a lot of fuel and make a lot of heat. Which seems entirely relevant to what the O.P. considers is happening (he said it acts like it's not getting fuel). And then states … any thoughts? And that's what we've done … added our thoughts.

                          So I'm not following what you're finding wrong with anything AJ or I have posted.
                           
                        • rumblefish360

                          rumblefish360 so close yet so far away

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                          Yup my bad, everyone cruises at 4K with 3.23’s and a 27 inch tire. It makes sense....:lol:
                           
                        • 6kevin8

                          6kevin8 Well-Known Member

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                          Thanks for the response. The issue was never really resolved. I just kinda ignored it. It would be somewhat troubling at times because I would be afraid of getting stranded, but luckily that didn't happen. I took the car to the Hot Rod 70th Anniversary at Pomona which was like a 45 minute drive for me and I made it there and back. I haven't really driven the car since though because the rear passenger shockplate broke off on the way home (DAMN YOU CHINA). I'll probably take the car up to my dad's house at some point during the winter, but its hard to touch it while in school and the apartment complex doesn't want you working on your car.
                           
                        • 6kevin8

                          6kevin8 Well-Known Member

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                          When blowing through the fuel filter, there isn't much resistance. I haven't checked the pump at all, I think it's the same one I had on the slant six before the swap. The gas tank is new I think, it came with the new body after my crash.
                           
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