1938 Evinrude 3 hp weak spark on one cylinder

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  • August 31, 2017 at 9:37 pm #8083
    olcah
    Participant

      US Member

      My Evinrude is model 4258, two cylinder, 3 HP. It has a single magneto coil and one set of points. At each end of the coil there is a connection to one of the spark plugs (two plugs, one connected to each end of the coil). It took a bad fall last year from my moving trailer (my fault) and I have been trying to get it to run since. It ran well before the accident. The starboard cylinder has very weak spark. Sometimes moving the spark advance lever will cause it to produce normal spark on that cylinder. The other cylinder has normal spark regardless of spark lever position.
      I cleaned and adjusted the points and replaced the old spark plug wires but no change, still weak or no spark on the starboard cylinder. I have looked for loose connections and so far haven’t found any.

      Could this be a symptom of a bad coil?

      August 31, 2017 at 10:48 pm #64109
      Buccaneer
      Participant

        US Member

        If that’s an opposed cylinder motor that both cylinders fire the same time,
        I can’t think of much wrong other than a bad coil. Might want to make sure
        the mag plate is snug and grounding well. Not sure how you were checking
        spark, but make sure you have two good spark plugs and that if your checking
        spark on one cylinder, that the other plug wire is grounded.

        Prepare to be boarded!

        September 1, 2017 at 2:53 pm #64129
        olcah
        Participant

          US Member

          Yes, it is an opposed twin. Plugs are near new and I swapped cylinders. Checked sparked with gap tester and with neon light, same result. Yes just one set of points. I am thinking coil too.

          September 1, 2017 at 3:43 pm #64131
          garry-in-michigan
          Participant

            Lifetime Member

            Normally the high voltage section of the coil is totally isolated from ground. The circuit is completed through the spark plugs when they are screwed into the cylinders. Voltage is supplied as needed. If one plug wire is grounded, the other should jump a 1/2" gap. If they show unequal voltages, the coil has become internally grounded and should be rewound or replaced. Since you have replaced the spark plug wires, no other explanation is logical. . . 😕

            September 1, 2017 at 4:12 pm #64133
            garry-in-michigan
            Participant

              Lifetime Member

              P.S. Dropping is not the most common reason for this failure. It is usually caused by cranking the motor with the plug wires disconnected and just dangling. Maximum voltage is generated with nowhere to go. It will eventually find the thinest place in the insolation and burrow through. To keep this from happening ALWAYS ground the plug wires when the spark plugs are removed. The motor should NEVER be cranked with the wires dangling.

              ALSO, In order to maintain the magnetism in those antique soft iron magnets, put a piece of iron (old wrench) across the magnetic poles in the flywheel whenever it is removed from the outboard. Need I remind you that this must be removed when the flywheel is installed ? 🙄

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