Why is my slanty chugging? (Problem solved, for now anyway)

Slant 6 Engines

  1. cruiser

    cruiser Well-Known Member

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    Hi All: Here's one for you expert slant six tuners. I have an interesting thing happening with my 225 in my 1974 Duster (torqueflite auto). First of all, I just love the motor. It runs great and has plenty of power for my car. I've restored it completely stock. The replacement 1974 model year motor has 100K miles on it and has good compression. The head and valve train were overhauled 5K miles ago. The exhaust system is totally stock, including the resonator which came automatically with the Noise Reduction Package. The carb is a rebuilt stock one barrel Holley 1945, stock choke setup. New clean NGK ZFR-5N plugs with gaskets removed, gapped to .035" as the factory shop manual recommends. Here's the problem, if it is in fact a problem at all. The car has a very nice butter smooth idle when warmed up. Idle RPM set to the factory recommended setting. If I stand by the exhaust pipe and listen to the exhaust, it "chugs" or "puffs" as it idles - maybe one or two chugs every ten seconds. The engine doesn't seem to slow down when it's going this. I've adjusted the idle mixture screw, which will increase or decrease the amount of "chugging", but I can never find that sweet spot where it completely goes away. I'm baffled. Do these individual "puffs" represent a failure of a cylinder to fire at that moment? if so, it doesn't seem to slow the engine down. Or is this "puffing" actually normal with a resonator equipped exhaust system, and it's not really missing at all. I just can't figure it out. Ideas? Slant Six Dan, if you're out there, please weigh in. Thanks, everybody!
     
  2. Murray

    Murray FABO Gold Member FABO Gold Member

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    How old are the ignition wires? I have seen wires that will occasionally arc even though they ohm out OK. This is due a breakdown in the insulation by antifreeze soaking. Try looking at your motor running at night to see it.
     
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    • Ian McCrimmon

      Ian McCrimmon Well-Known Member

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      Chugging usually indicates a blockage in the exhaust system.
       
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      • Ironracer

        Ironracer Gearhead

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        Mine always chugged right along, like a Great Lil machine!
         
      • MOPAROFFICIAL

        MOPAROFFICIAL FABO Gold Member FABO Gold Member

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        Keep an eye on the exh valves burning on no.4/5.

        Could be muffler collapsing inside.

        Does it suck a peice of paper flat at the tail pipe when running?
         
      • halfafish

        halfafish Damn those rabbits, and their holes! FABO Gold Member

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        If you want to page slantsixdan, it's done like this: @slantsixdan !!
         
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        • slantsixdan

          slantsixdan =..=

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          Yes. That splutter/chuff/pluff sound is a single-cylinder misfire. A less primitive ignition system, aside from its other benefits, will likely reduce or substantially eliminate the misfiring—HEI upgrade with attendant upgrades to the spark plug wires and a carefully optimized distributor cap and rotor (per the suggestions in this long but worthwhile thread).

          But keep in mind we are talking about a car most of 50 years old, with a not-very-well-designed carburetor known, especially in its first couple years ('74-'75) for poor mixture distribution. If the only symptom is the audible misfiring, you'd be perfectly well within reason to just quit putting your ear near the tailpipe.
           
          Last edited: Oct 26, 2021
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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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            This ^^^
            indicates a problem with an exhaust valve not closing.
            But I'll bet you are experiencing an explosion in the exhaust system, due to air getting into it, usually up near the manifold or head.
             
          • cruiser

            cruiser Well-Known Member

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            Ignition wires are new.
             
          • cruiser

            cruiser Well-Known Member

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            Dan - the audible misfire at idle is the only symptom. The engine runs really well at highway speeds. If it's misfiring on the highway, I certainly cannot tell. Maybe I'm just thinking about it too much. Your point about a nearly fifty year old car is well taken.
             
          • cruiser

            cruiser Well-Known Member

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            Tried that, and no it doesn't suck the piece of paper. Thanks - BOB
             
          • MOPAROFFICIAL

            MOPAROFFICIAL FABO Gold Member FABO Gold Member

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            Bad fuel distribution or incorrect a/f.
            Be it Carb or intake.
            Unless the spark or timing is unstable...it should fire every cyl like clockwork with a stock cam.
             
          • RustyRatRod

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

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            It's entirely possible the car did it when new. My 170 did it. I just drove heck out of it.
             
          • TF360

            TF360 Well-Known Member

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            I would get rid of that factory exhaust, and go with 2 1/4 pipe. She will run even better. Maybe it's a little vacuum leak
             
          • cruiser

            cruiser Well-Known Member

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            Okay, I just replaced all six plugs with new NGK ZFR-5N plugs, and the problem went away.This car does seem to go through a lot of spark plugs. My maintenance records that came with the car (going back to 1973) reveal the same thing. Lots of spark plug changes. Anyhow, thanks to all who weighed in on the problem - it's much appreciated!
             
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            • Dartswinger70

              Dartswinger70 I been North I been East to the California beach

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              How often are you changing the plugs? Used to be every 12 thousand miles back in the day I'm thinking...I remember my mom changing points/plugs every fall for winter in Pa on a points car 66 Chevelle with 283 .
               
            • AJ/FormS

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

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              the plugs on my HO367 were new in 1999, over 100,000 miles ago. Car still "misses" same as it always has...... lol, due to the size of the cam no doubt..............
              @cruiser take a good look at the porcelains of those plugs; I'll bet you will find a cracked insulator. It may look like a hairline pencil track.
              When that happens, then at idle, the spark can leak to ground, but when you increase the load and/or rpm, two things happen;
              1) the spark energy is increased, and
              2) the time to leak is reduced.
              So it is entirely possible for this to fit your symptoms.
              As the crack gets larger, and fills with soot, the symptoms get worse, and eventually, if left too long, that porcelain my rupture and cast off very hard glass bits..
              The crack is usually caused by detonation, so if this is happening on an on-going basis, you better review your ignition timing program, or your lugging habits.
              But good news, it may not be a crack at all, but just a carbon-track. You'll have to figure out which it is.
               
            • slantsixdan

              slantsixdan =..=

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              15k miles with leaded gasoline, 30k with unleaded was the standard in the '70s. We have better spark plugs now, and cleaner-burning gasoline, so you should be able to go a long time on a set of plugs. If putting in new plugs stopped the misfire, it reinforces that upgrading your ignition system (as a whole) will likely knock down the car's seeming need for brand-new plugs to run without misfire.
               
              Last edited: Oct 26, 2021
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              • cruiser

                cruiser Well-Known Member

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                Thanks Dan!
                 
              • Bewy

                Bewy Well-Known Member

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                Plugs might be getting carbon coated from a rich mixture or poor oil control in the chamber. Plugs then misfire & new plugs 'cure' the problem.
                 
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                • volaredon

                  volaredon Well-Known Member

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                  Well, it ain't like plugs for these engines are all that expensive or all that tough to put in, so what's the big deal? I change mine even on my newer vehicles every 30k miles whether they need it or not.(1 each... 3.9, 318 magnum,360 magnum)
                   
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