Home Forum Ask A Member 1956 Johnson 30hp Only 1 Cylinder Firing

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    Forum Wizards:  The good news is that it has been quite a while since I had to past a question here.  The bad news is that I need to now!

    Here is the situation:  Only 1 cylinder in the motor is firing resulting in sluggish (basically half) performance.  Here is the history of the situation:

    1. Took boat and it operated for about 8-10 miles able to go at full speed (24MPH) with no issues. Start to head back to boat launch and suddenly top speed is like 8 MPH.  So we puttered back for as long as it took.  Figured it was a spark plug issue so took boat out of water, changed spark plug, put back in and it ran fine at 24 MPH.  The one spark plug I did notice was bone dry at that time which caught my attention…
    2. Boat in garage for about 6 weeks…then take to another lake, put in water and back to running at 8-10 MPH.  By this time I had ordered a 10 pack of spark plugs.  Since I was at a boat show, decided I would put 2 new plugs in and giver her a try – same thing, half the performance.
    3. Attached is the picture of the spark plugs I just took out. The left one is from the upper chamber and seems very dry comapred to the right one (bottom chamber) that looks wet which is what I expect.
    4. I also have attached a picture of my fuel pump which this forum led me to which was a gamechanger as far as performance this past year.  That thread I started on that issue can be found here: Topic: 1956 Johnson 30hp High Speed Tuning – Antique Outboard Motor Club,Inc (aomci.org)

    Any thoughts on how I can attempt to triage the situation?  The pump is serviceable and did come with some spare parts.  Or perhaps a clogged fuel line?

    Many thanks!



      US Member

      do you have spark?
      is the wet plug on the fuel pump?
      dry plug is fine.
      could be points coil condenser or plug wire on the fouling cyl.


      Is my easiest way to check spark to use the manual pull with the plug out (I assume so – but have not tried!)


        US Member



          Canada Member - 2 Years

          if sparks are ok  ..could be a rupture diaphragm leaking extra fuel in the crankcase

          switch sparkplugs    maybe  the wet one is bad…it happens !



          Joining AOMCI has priviledges 🙂


            US Member

            Must ground the base of the spark plug to the block while you pull start to check spark.

            1 user thanked author for this post.

            OK – I will go back and try.  I did not do this and saw no spark from either which befuddled me.  I’ll report back after I do this properly – just need my son to roll out of bed… 🙂


            OK – was able to check both spark plugs against the engine block and they both were producing a spark.


            Now that I have confirmed that the sparkplugs are working, any suggestions on a next set of things to explore?

            Many thanks for all the help!


              US Member

              point gap correctly is the cam on upside down?

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