Rough idle... only when warm.

Mopar Performance Issues

  1. Jonathan Bass

    Jonathan Bass Member

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    Been trying to get ideas on what people think this issue could be. I'm lost at this point.

    When cold. Car idles, shifts from park smooth, and runs well.

    When warm. Idles rough, engine shaking, shifts from park hard, performance loss.

    I don't understand what would cause that difference other than the choke. Which seems to be adjusted and working properly. Anyone have ideas other than that?

    No amount of timing or idle / fuel mixture adjustment while car is warm fixes the issue.

    Changing from manifold to ported vacuum changes nothing either.

    I sprayed around for more vacuum. Found nothing. Reseated the intake manifold and carburetor. Made sure the throttle blades were closed properly in carburetor. Replaced hosing. Readjusted mixture and idle. Switched from manifold to ported vacuum and retimed. Issue still persists ONLY WHEN WARM.

    I read on a classic Mustang forum somewhere that someone was having a similar issue and replaced their PCV valve and that fixed it. I ordered a new one, so I'll test that when I can.

    But otherwise. Does anybody have any ideas? I'm frustrated.
     
  2. Joey4speed

    Joey4speed FABO Gold Member FABO Gold Member

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    Sounds like you covered all things vacuum.
    What year, make, engine?
    Curious, If you simply place your hand over the carb when its warm, as to choke it, does the idle improve?
    Joe
     
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    • Professor Fate

      Professor Fate Push the button, Max...

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      Vacuum line should be disconnected from the advance cannister and hose plugged when setting timing.
       
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      • Tooljunkie

        Tooljunkie King of cobble/master of the broken bolt FABO Gold Member

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        Details man! power brakes? Slant 6 or a hemi? Somewhere in between?
        power brakes my have a booster leaking vacuum.
        Had a dirty carb, would run not bad until it warmed up,choke would come off and it would get rough. Pulled carb and cleaned it. Solved!
        Plugs burning clean?
        What do you have for a carb?
         
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        • Jonathan Bass

          Jonathan Bass Member

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          361/727. Power brakes yes, but I disconnected the brakes to eliminate that issue. Pretty much brand new Edelbrock 1411, I got it about 3-4 months ago. Choking the carburetor does not improve idle.
           
        • Jonathan Bass

          Jonathan Bass Member

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          The thing that confuses me most is. Why would the transmission shift from park harder at lower RPM when I'm having warm rough idle, than it would at a cold start with higher fast idle RPM?

          Shifts smooth first start up of day. Shifts hard when warm.
           
        • Tooljunkie

          Tooljunkie King of cobble/master of the broken bolt FABO Gold Member

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          Motor mount. Trans mount. Cable shifter? Cable routed too close to exhaust. Been there...
           
        • Joey4speed

          Joey4speed FABO Gold Member FABO Gold Member

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          Jonathan,
          After installing a brand new Edelbrock last year. I had an
          Intermittent backfire and ran hot, thought it was timing or a secondary ignition problem. Tried everything with no luck.
          A wise ole mechanic said the carb was no good, he "didnt care if it was new".
          Had it rebuilt, never ran better, smoother, cooler and stronger. Guessing it had an internal Lean issue.
          Who knew?
           
        • dano

          dano Evil Handy Man

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          Is the fuel boiling in the bowls? Did you put a heat isolator between the engine and carburetor? Have you verified the float level and fuel pressure?
           
          Last edited: Jan 3, 2021
        • pishta

          pishta I know I'm right....

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          Carterbrocks run hot. Use a phenolic/plastic/wood or that 1/4 fiber gasket between the intake an the carb. when choked it changes the fuel circuit. Kinda sounds like your float level may be very low. Choked creates alot more vacuum so it pulls more fuel, if your float level is low, your fuel pressure to the boosters is going to be lower and its gonna run lean. When i starts to run like crap, shut it down and pull the top of the carb off as fast as you can to see what the fuel level is.
           
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          • Jonathan Bass

            Jonathan Bass Member

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            I do not have a carb spacer... just a gasket and adapter plate. I can get a spacer and heat insulator gasket and see what that does.
             
          • Jonathan Bass

            Jonathan Bass Member

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            If I give it VERY light throttle with foot on brake while in drive, the rough shaking idle will smooth out... would that be an indication of anything? Increasing idle speed does not achieve same affect.
             
          • Kendog 170

            Kendog 170 Let the boy go !

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            Definitly get the insulated gasket even if this isn't your issue. You have to have one on an Eddy carb.
             
          • halifaxhops

            halifaxhops It's going to get stupid around here! FABO Gold Member

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            What ignition you have? Also does it get progressively worse as it warms up?
             
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            • mbaird

              mbaird mbaird

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              My brother had a similar problem on his Firebird . Turned out to be the fuel pump. When hot it couldnt keep up .
              Worth checking fuel pressure cold and hot ....
               
            • Jonathan Bass

              Jonathan Bass Member

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              Trying the spacer and gasket today.

              I have a new MSD Blaster coil, new ballast resistor on firewall, new voltage regulator, and original distributor that it had when I bought the car.

              Problem existed with previous coil, ballast, and regulator.

              Will check fuel pressure.
               
            • swedefish

              swedefish Well-Known Member

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              Step up springs in carb?
               
            • TrailBeast

              TrailBeast AKA Mopars4us on Youtube

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              Cold fluid flows slower than hot fluid. (soft shifts when cold) and trying to compensate with idle speed because of a rough running motor can cause harsh shifts, so for now just deal with the running quality and the shift engagement issue will probably go away.

              Sounds to me like the carb is going lean when the choke comes off as it warms up.
              As mentioned above once its warmed up and running crappy try using your hand to cover part of the top of the carb to restrict the air going in.
              If it runs better as you restrict it tells you it’s lean.
              It also works to point out a rich condition, as the restriction causes the motor to run worse.
               
            • pishta

              pishta I know I'm right....

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              • Jonathan Bass

                Jonathan Bass Member

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                Also yes it progressively gets worse and worse as I continue driving.

                It hasn't shut off yet.
                 
              • halifaxhops

                halifaxhops It's going to get stupid around here! FABO Gold Member

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                OK what ignition? Points, electronic, what make etc. Can be a component on its way out.
                 
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                • Jonathan Bass

                  Jonathan Bass Member

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                  MSD Blaster 2. Brand new. Same issue with last coil. New ballast, new voltage regulator. Only thing stock is distributor. Going to replace that soon. Upgrading to electronic.
                   
                • halifaxhops

                  halifaxhops It's going to get stupid around here! FABO Gold Member

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                  OK if it is points prob a condenser on its way out and if it is electronic can be a pickup in it. You have to be more specific so we can help.
                   
                • AJ/FormS

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

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                  what's yur Idle timing?
                   
                • Bewy

                  Bewy Well-Known Member

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                  Could be a few things:
                  [1] Lifter preload [ bottomed out ]. Unlikely but possible. As the engine heats up & parts expand, bottomed out lifters have nowhere to go & stop the valves from seating.
                  [2] Carb. Remove it & disassemble. Unlike brand H & clones, you won't need gaskets if you are careful. Blow out passages with compressed air. In the primary clusters, you will see a thick & thin brass tube hanging out the bottom. The pinched lower end of the thin tube is the idle fuel feed. Poke a piece of wire in it to make sure it is not blocked. Be careful, the boosters have a L & R, not interchangeable. Re-assemble with 7/16" float setting. Set idle mixture screws 2.5-3 turns out from seated.
                  With the carb back on the car, loosen the tops [ 2 turns on the screws ] on the metering rod pistons. Swivel the tops slightly so you can see the piston but it remains captured. Tighten screws.
                  Warm up engine & adjust mixture screws [ in gear if auto ] for smoothest/highest idle speed. Look at the pistons; they should be down & not moving. If they are moving, you need softer step up springs. If they are moving, use a thin screw driver or wire to push the pistons down. If the engine note changes, it means the pri throttle blades are open too far at idle. This can be caused by a big cam, worn cam, PCV that is oscillating & not closing [ usually caused by low vacuum from big cam ].
                   
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