1956 Johnson 3HP compression

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  • May 24, 2025 at 10:44 am #296721
    tibor

      Based on my experience working on the two old Naptune magnetos, it is worth checking that you do not have an accidental short to ground on the condenser or the coil primary. Since you are getting some sort of spark on the secondary, it is unlikely to have a short on the primary but it takes a minute to check.

      You should read the 1.5 Ohm (which as far as I know is good) when the points are open and a short (0 Ohm) when the points are closed. The beeping short testing does not work because most multimeters will beep at anything less than 50 Ohm.

      You could also unscrew the coil primary (and condenser) and the then the points should read disconnect or short as you turn the crank (Close to 8K Ohm on the secondary should also be good.)

      If you have an inductance tester, you can check the coils on their laminate core, I was getting between 0.7 and 3 milliHenry on the primaries and 8-9 Henry on the secondaries.

      Is your motor one where the coils can be moved closer to the magnets? It may be worth checking the gap between the laminate core and the magnets then.

       

      May 24, 2025 at 11:38 am #296732
      Tacitus043

        I would love to get a meter than does capacitance and inductance.  I’ve seen some inexpensive ones on Amazon, but I don’t know if I trust them.  I don’t think the wife would be too happy if I went out and bought a Fluke.

        The coils are adjustable, and I didn’t have the special ring to align them.  I saw videos of some guys more or less eyeballing them into place with the mounting bosses on the mag plate and it seemed to work for them, so that’s what I did.  I did order the alignment ring, so when that finally comes in I will be sure to set the coils with that.

        For kicks I put an inductive pickup on the secondary wire and measured voltage.  It only spiked to around 6kV.  I estimate it needs closer to 20kV.  Once I get it running I’ll measure it again and see what a healthy spark looks like on the scope.

        May 24, 2025 at 12:13 pm #296739
        crosbyman
        Participant

          Canada Member

          see articles 1-4 on condensers- capacitors testing and repairs     Amazon Capacitance meters / VOM are excellent for basic ohms and capacitance value testing  but do not tell you  if any good under actual load conditions..  VOM/Cap meters  are internally  powered with 1.5 to 9v    batteries  they are  but insufficient to confirm  if condensers are any good  under real world  load and heat conditions

          Magazine Articles | Western Reserve Chapter

          Joining AOMCI has priviledges 🙂

          May 24, 2025 at 4:40 pm #296742
          tibor

            Tacitus043, the one I have is a $30 LCR meter from Amazon. It will measure inductance and capacitance with the limit that crosbyman highlighted, from a supply of ~9 Volts. I don’t think that for hobbyists a Fluke makes sense. (I am saying that as I do have a Fluke but I seem to use my cheap one all the time.) Fluke is great if you are a large company and want to have the same product supported for a decade unchanged.

            I am curious, how did you measure the secondary voltage with an inductive pickup?

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