6 hp Johnson no fire

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  • speedlinerbu

    US Member - 1 Year (includes $3 online payment fee)
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    #204030

    Why would my 1965 Johnson 6 hp not run on the lower cylinder?
    A spark plug check shows it is firing, but the cylinder is running cold.


    fleetwin

    US Member - 2 Years
    Replies: 4737
    Topics: 46
    #204031

    What do you mean by “a spark plug check”? Are you sure the lower cylinder plug is actually firing under compression? I’m assuming you have tried new plugs, how did you check for spark?


    speedlinerbu

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 23
    Topics: 15
    #204034

    Engine ran fine last summer. plugs were new then.
    Today, when the lower cylinder obviously was the one not running. Cold to the touch. I pulled both plugs and grounded them against the block.
    Both of them fired.
    If lower ignition set will not fire plug under compression how do I tell? And why would that happen in the first place? Thanks for your input.


    mercuryman


    Replies: 467
    Topics: 167
    #204035

    The ANTIQUE OUTBOARDER for Apri 2020l has a good article ” Spark Plug Voltage Requirements read this and you understand the difference between SPARK and GOOD Spark.

    Moat of the time cleaning the point will fix it.

    • This reply was modified 3 years, 10 months ago by mercuryman.

    fleetwin

    US Member - 2 Years
    Replies: 4737
    Topics: 46
    #204045

    You can try putting one of those inexpensive “inline neon spark testers” in between the spark plug and ignition wire while the engine is running. You can get these at many auto parts stores inexpensively. Not a fool proof test, but can be helpful… Try it on the top cylinder first so you will know how intense/frequent the neon light should be…It is best to do this test when the sun is going down. Be sure the connections don’t accidentally get grounded to the block. Just because the plugs are new, doesn’t mean they are any good…Try swapping the plugs to see if the upper cylinder stops firing with the lower cylinder’s plug installed.
    To answer your question, the ignition system may have enough power to fire a plug when grounded to the block, but it takes considerably more energy/voltage to fire it in the combustion chamber under compression….


    outbdnut2

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 1705
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    #204049

    See if the spark will jump about 1/4 inch in free air. You can take an old spark plug and bend the gap open farther.

    Does that cylinder have compression? If it’s really low You could have broken rings, or a blown head gasket and it will not fire. If you don’t have a compression gauge, slowly pull the starter rope and see if compression “feels” about the same in both cylinders.

    Another problem could be a broken or bent reed.

    Let us know what you find!
    Dave


    speedlinerbu

    US Member - 1 Year (includes $3 online payment fee)
    Replies: 23
    Topics: 15
    #204629

    Thanks for all the contributions. I changed the coil and things are better. But not 100% in test bucktet.
    Need to get it down to the river to find out more.

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