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Rough idle

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  • Rough idle

    Hello Everyone!

    My 1957 Silver Hawk has a rough idle. Compression test shows 4 cylinders at 125 to 130 and 2 at about 100(the 2nd and third back from the front)(champion 185 engine). I have replaced the plugs , wires, rotor button, and installed a Pertronics ignition. Can the two low cylinders cause the rough idle?? The cars runs out real nice on the road--pulls nicely. It's mainly when it's in gear and stopped. I put a little oil down the plugs holes and the low cylinders jumped up to 130 pounds. From what I've read, this means "rings"? I know valves are the other possibility, but I'm not sure if the lower compression in those two cylinders will cause this. I would think so cause your not "pushing" as hard with those two pistons? Can anyone chime in here? As alwas, Thanks----Dan

  • #2
    Hi, Dan,

    The two weak cylinders shouldn't cause a really rough idle.

    You have done a first-class ignition tuneup, plugs, wires, points, condensor, rotor, cap, timing and dwell.

    IIRC, there is a screw-in plug in the intake, left over from the vacuum wiper days. Using a good vacuum gauge, engine fully warmed, brake on, transmission in gear, adjust the idle mixture screw to give the highest vacuum reading.

    Finally, does your Champion burn oil? Most high-mileage Champs have worn valve guides. Under high vacuum conditions, such as idle, the engine is sucking vapor up the intake guides and actually bypassing the carburetor. This can cause a rough idle which cannot be fully compensated by the carb adjustments.

    thnx, jack vines.



    • #3
      You might also check the valve clearance.


      • #4
        Thanks to all---

        PackardV8-----It does not burn too much oil---I will check out the screw in plug in the intake---Thanks

        John----I have one valve that "clicks" a little more than I would like. I will take a closer look at the valves as well---Thanks


        • #5
          You might take a look at the carb, has it been rebuilt recently?

          When I rebuilt that Stromburg carb years ago one of the things I noticed was how full of junk one of the tubes was (it had something to do with the idle circuit, but that was 30 years ago so some of the details are a bit fuzzy!). In any case the car idled OK but not great before the rebuild, afterward it idled beautifully.

          Jeff DeWitt