Home Forum Ask A Member 1956 Johnson 3HP compression

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  • #296629

    I finally got my 56′ Johnson 3HP back together and am trying to get it fired up.  Spark tester shows spark on each cylinder, and it seems to be getting fuel.  Every couple pulls I get a decent pop or two from the exhaust.  It definitely sounds like she wants light.  The compression gauge shows 50 psi on each cylinder, with it taking about three pulls to max out.  50 seems pretty low to me.  I was hoping to see closer to 80.  Is 50 low enough to prevent it from firing up?

    #296645
    seakaye12
    Participant

      US Member

      Yes that’s low but might just be your tester.  Can you compare on another similar motor that runs?

      Sometimes a low compression motor will start when spun with a drill.  If you succeed you can hope that the rings will free up after a bit of run time and things will improve….

      #296646
      fleetwin
      Participant

        US Member

        Are you sure the plug wires are not swapped?   It is easy to get them mixed up

        #296673

        Well, I think I may have had the plug wires backwards.  I had the wire from the coil closest to the carb going to the bottom cylinder.  Correct me if I’m wrong but the coil closest to the carb should go to the #1 cylinder (top)?

        After switching the plug wires still no start, or popping.  It’s worth noting that I tested spark initially with one of those inline testers that illuminate a small bulb.  I was able to get my adjustable air gap tester from work and test it this morning and low and behold I cannot get a spark to jump a 1/4″ gap.  The spark is there, but seems weak.  The Chinese coils I have in the motor each have 8.8k ohms on the secondaries and 1.5 ohm on the primaries.  Thoughts?  Thanks!

        #296674
        David Bartlett
        Participant

          US Member

          Did you replace the condensers when you changed the coils? Points clean and shiny? Also, you probably changed the plug wires with the new coils, but????

          New plugs?

           

          David Bartlett
          Pine Tree Boating Club Chapter

          "I don't fully understand everything I know!"

          #296675

          Brand new points and condensers.  New copper core plug wires and plugs.  Dressed the new points with fine sand paper and set to .020.

          #296677
          crosbyman
          Participant

            Canada Member

            Correct me if I’m wrong but the coil closest to the carb should go to the #1 cylinder (top)?

            YES… front coil to top plug.

            If you plan  working oldies why not get the  $20 Johnson  service manual  where everything is explained from top to bottom !

            https://watercraftmanuals.com/outboard/johnson/manuals/johnson-302231.htm

             

            Joining AOMCI has priviledges 🙂

            #296680
            BRENT BERGEN
            Participant

              US Member

              50 PSI is too low for the motor to run. If you check the compression with a known good compression tester, and it’s still around 50 PSI, then the best bet may be to find another used powerhead with better compression readings. There are many of these powerheads out there and other years that are interchangeable.

              Regarding the ignition, it might pay to go back and take a second look at the points. Once I used Sierra points, and after running the motor, the points were then misaligned. They were of poor quality and had to be trashed. Double check the points cam to ensure that it’s not installed upside down.

              Improvise-Adapt-Overcome

              #296681
              wedgie
              Participant

                It is a good idea to lap the cylinder cover  on a  thick plate of glass with some wet dry sandpaper, using a figure eight motion.

                The cover tends to warp creating a leak between top and bottom cylinder ,past the gasket. A symptom of this is identical psi on the gauge.

                #296697

                Thanks for the info guys.  Looks like I’ll have to pop off the flywheel and investigate the mag assembly.  Compression aside, the spark should be way stronger.  Hopefully my compression gauge is just reading low.  I’ve always used it to find relative differences between cylinders on bigger engines with higher compression and have not really paid attention to the overall number.

                I figure once I get the spark straight I’ll re-test, and hopefully she will fully light off.  If not I guess it’s back to the drawing board with the power head!

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