1939 Johnson 5hp AT-39 Running Hot

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  • July 16, 2024 at 8:42 pm #289211
    Swarlos
    Participant

      Hey folks, my grandfather’s old 5 horse motor gets really hot.  Earlier this year I fixed the water pump plunger because it had worn/corroded a small hole in the bottom that was allowing corn head grease up through the gearcase into the water pump.  I took the pump apart and cleaned everything as best I could, but I couldn’t get far up the water passages.  I know water is pumping through because my grandfather had a friend who was an outboard mechanic drill a small pee hole in a fitting in the water line under the head.  The water that comes through there gets really hot.  I haven’t measured the temperature of that water, but it’s hotter than any water I would wash my hands with.  I have a infrared thermometer that’s a little difficult to use while underway, but I’m getting readings on the head cover with the handle on it over 160 degrees.  I’m hesitant to pull the head off and look inside because I don’t know if I’d ever be able to replace whatever head gasket there is.  I was wondering if I could put some kind of solution into a barrel that would clean out the water passages without doing any damage.  Any feedback would be appreciated, I’m happy to provide further information if necessary.  Thanks in advance for any advice!

      July 17, 2024 at 9:22 am #289215
      The Boat House
      Participant

        That’s a special motor. Did gramps buy the motor when it was new? I’m not familiar with your powerhead but do have an LS-37 with the same lower unite. I don’t think you have a problem. You only have a piston pump trying to cool a 2 cylinder motor. They just can’t move the same amount of water an impeller can. The fact that the water is hot indicates its removing the heat from the cylinders. Cars use to have 160 degree thermostats in them. Today they’re over 200 degree. Although you can’t go back in time, you are having the same experience your grandfather did when running that motor. Tubs
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        Swarlos
        July 17, 2024 at 7:21 pm #289223
        Swarlos
        Participant

          Thanks for the reply Boat House.  I know he’s had it since he was young, so I guess his dad bought it for him new.  I’ll have to ask him.  It hasn’t seen much use these last few decades, but it would give me a smile to get it back into proper working order.  It’s a project that I’ve taken up for a few weeks every year or two for the last 6 years.  It seems to be running pretty well now, but I’m still nervous about opening it all the way up.  Since there isn’t any kind of high temperature alarm, how would I know if it’s getting too hot?

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