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  1. #1
    skyboy's Avatar
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    Plug gap closing while running.....?

    What are some things that could cause a spark plug gap to close to zero while the engine is running?

    700 engine, running normally, starts losing power then will only run on one cylinder. Removed spark plug on offending cylinder, gap nonexistent. No evidence of trauma to plug tip.

    Had the same exact thing happen on another ski with the same type engine. Running fine, spark plug gap closes to zero.

    In both cases, reset gap to proper value, fires up, runs great and so far not able to duplicate problem.


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    If you are using the correct length spark plugs then a possible cause is that the connecting rod bearings are failing. This allows the piston to rise up farther than normal due to excessive slop in the connecting rod.

    The piston hits the spark plug and closes the gap.

    I suggest you not run the engine any more until you inspect the engine internals.

    Another possibility is that some metal pieces are breaking off inside the engine and traveling up through the combustion chamber. Some of these pieces may hit the spark plug and close the gap. Inspection of the engine internals will tell you for sure.

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  4. #3
    skyboy's Avatar
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    Everything is stock and the plugs are BPR8ES. The first time it happened I was very concerned so I looked inside the combustion chamber with a dental camera (poor boy borescope). There was no buildup of carbon, FOD damage or other evidence of interference between piston and spark plug electrode. Surmise mechanical failure of that magnitude would be accompanied by other abnormal indications as well. The plug was re-gapped and the engine has run excellent since this happened (early spring).

    Then it happened again about 3 weeks ago on a different ski. This ski has a strong running, low time engine. No evidence of mechanical failure, plug re-gapped and engine has run great since.

    The other day, I was talking to a neighbor and he said it's probably bad fuel causing it, specifically ethanol blended fuel and water entrainment. I have never heard of this before (of course that doesn't mean anything)? I do buy gas for the skis at the same location, often from the same pump. When a transfer can is used, it's the same container stored in the same location. For full disclosure, the bad fuel info is coming from an engineer and his colleagues at an auto company. If it would not for that and my fueling habits, I'd dismiss the bad fuel scenario.

    I'm curious how likely gasoline, or the way I am handling/storing it is the cause? Apparently this is not very common so it's entirely possible I am doing something wrong?

  5. #4
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    piston won't hit the plug electrode,it'll strike the dome way before getting anywhere close to the plug.if a crank bearing comes apart or the wrist pin bearing the material can cause this,bruce from wet wolf said that a lean condition can cause it.the electrode gets hot enough to bend.have you replaced the fuel lines?looked at the carb internal screens? i'd start there.

  6. #5
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by TxPro1200 View Post
    piston won't hit the plug electrode,it'll strike the dome way before getting anywhere close to the plug.
    Absolutely correct.

    Stock piston, stock head, stock plug = impossible for the piston to close the gap.

    Heat-yes
    Broken pieces of anything-yes

    A full, unbroken piston just rising in height - No.
    Last edited by CrazyA; 01-11-2015 at 12:38 PM.

  7. #6
    rpa101's Avatar
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    Heat and contact will close a plug gap bpr plugs have longer electrodes witch are susceptible to detonation
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  8. #7
    skyboy's Avatar
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    I have the BPR plugs because that was the original spec. I'd gladly try something different provided it doesn't cause hard starting or other operating problems. Certainly don't want the ground electrode to break off from too much bending and/or heat.

    The ski I was riding when a plug malfunctioned was running at low power and if anything, that engine runs on the rich side. I wasn't on the other ski when the plug shorted out but that cylinder is definitely not running lean. It could probably use a timing check so I'll do that before the next outing. Carbs recently cleaned/checked and all fuel lines less than 2 yrs old.

    Would there be any benefit by going up to 92+ grade fuel?

  9. #8

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    had this happen on my Pro 785's found out was water leaking into the cylinder, remember water will not compress, hence bending the metal electrode

  10. #9
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by skyboy View Post
    I have the BPR plugs because that was the original spec.

    ...

    Would there be any benefit by going up to 92+ grade fuel?
    Certainly switching to a higher octane fuel is an easy change to make. If the problem goes away then it gives you clues as to the underlying cause(s).

  11. #10

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    This is a somewhat related observation and may give you some ideas. I ran a Pro1200 for 11 seasons which also has a domestic engine (I believe it is a 700 with another cylinder added). The problem I had infrequently is the ground electrode would break off. My theory (backed by over 300 hours of use) was that there is a harmonic vibration in the crank assembly at a certain RPM. If I held it at that RPM range for any length of time, a ground electrode would break. Surprisingly, the performance wasn't affected even though the piston suffered massive scarring from the electrode piece bouncing around before being sent out the exhaust port.

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