spark plug

Discussion in 'Outboards' started by valvebounce, Jun 25, 2019.

  1. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    One of my plugs has no continuity through it.When the engine starts it ticks over,but when I rev it,it picks up,misfires and then gets over the misfire.Is it possible for a plug to cut in as the revs increase?
    The price of a set of plugs is no problem,(I am upgrading to iridium plugs)
    It would be handy to know about the plug for future reference.
     
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Have you checked the coil/coils?
     
  3. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    It is possible for the electrode to be broken, but the spark jumps the gap. Are these resistor plugs?
     
  4. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    I haven't checked the coils.
    I don't think the plugs are the resistor type (NGK B9HS)
    Like I mentioned,I got a circuit through the main electrode on one,but not the other,maybe there is a break in the plug core.
    It's a 1999 25hp Johnson,two cylinder,2 coils,a starter coil and a power pack type ignition.
     
  5. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Why would there be continuity through a spark plug?
    My experience with iridium plugs has been poor and evidently a major marine shop told me to stay away from them . Last spring put the Mastercraft back in the water, ran across the lake, no problem.
    Next day, started idling out from the dock, an error code came up, faulty injector, Ilmar engine, prepared myself for a 4 figure repair. The marine mechanic phoned me the next day said it was
    ready and would be about $100 bucks!!!
    Plugs failed, Iridium Plugs, the well advertised expensive ones, he said that they have this issue all the time with Iridium plugs.
     
  6. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    Hi Barry,the centre electrode in the plug should have continuity I think.
    Nice to find out Marine engineers don't approve of Iridium plugs,I had a Volvo with them in(car)
    and they lasted for a long time.
    I must say though,no one seems to use them on outboards.
     
  7. Barry
    Joined: Mar 2002
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    Barry Senior Member


    Your original post mentioned continuity of the plug. I took this to mean that you went from the outside electrode, to the core electrode. Obviously there is continuity from the top of the plug, the terminal to the inside ceramic surrounded tip. On any plug with an R, a resistor plug, the resistance could be in the 5000 - 6000 ohm maybe higher and without an R, open circuit

    Your other question in the first post, "can a plug begin to work at higher rpm"
    If the voltage coming off the mag on an outboard is not high enough for the voltage to jump the gap then there will be no spark. At low rpm, you may not get enough voltage to bridge the gap, but with higher rpm, your voltage may go up, making the plug "cut in"

    Of possible interest, Air has an electrical resistance that resists open spark travel. For air it is about 75,000V needed to jump a 1 inch gap, 2,250V to jump a .030" spark plug gap.
    So if your plug is not firing at low rpm, perhaps you do not have enough voltage available until the rpm picks up.
    I was unable to find an value for the voltage needed to jump a gasoline fuel mixture when compressed, one might suspect lower.
     
  8. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    Looks like its been a compound fault,and the plug hasn't helped.
    We have some good weather on the way in the next few days,so I'll be out there sorting it out.
    I've got new plugs,new fuel pipe and clips.
    This engine has not been on the boat yet,the cables were 2ft too short so I will need to fit longer cables.
    With a bit of luck,the steering won't be a problem.
    I'll get back on here and let you know how I get on."V"
     
  9. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Compressed air has a higher resistance. The old school sparkplug testers would show at what pressure the plug would stop sparking. They had a little glass window to see what was happening.
     
  10. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Not sure on compressed air, I was thinking that the higher hydrocarbon presence, mixed with droplets of wet fuel might lower the resistance
     
  11. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The mixture in the cylinder is mainly air. The ignition is timed to fire at approximately maximum pressure, which is about 150 PSI.
     

  12. valvebounce
    Joined: Dec 2010
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    Location: manchester uk

    valvebounce Senior Member

    I've had problems with the NGK 'R' plugs down at the motocross track a few years back.
    They barely lasted 3hrs.We swapped over to Platinum plugs,(Bosch)and had hours more
    out of them.
    The guy at the motocross shop told me the 'R' stood for Racing at the time.He caused us a few
    early finishes for a short while.
    Ever since,I check plugs out for the specs.
     
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