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  1. #1
    mrbtd's Avatar
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    Help with failure diagnosis

    Lost my #1 cylinder late in the day Saturday @ MB (timing worked out ok).

    Boat ran very hard for thee straight days after a dilligent break in. Put 10 hours on the boat before this happened.

    Compression was 160 in all three. Ran stock ignition for break in. Used nothing but premium fuel.

    There is no question I needed more prop (was spinning a 14/20 @7310) but I was having so much fun I didnt want to mess with anything.

    After LONG idle periods it did load up a bit and I plan to clean that up with the low speed adjustment screws.

    I was at 5K RPM's for about 4 miles and felt a little something and though to myself - I shouldnt be @ 5K. Not more than 30 seconds later it seized and motor shut down.

    Pistons were coated (maxpower). The cylinder may only need to be honed.

    Did the lean spot in the carbs get me ? If so why only on #1 wash for #2 is not bad.

    If I had it to do over I would of retorqued after a few heat cycles. Just by feel the head bolts were a little loose (R&D three piece). I checked torque for the #2 and I got MAYBE a 1/4 turn on each bolt.

    The head has a few dings but I am confident I can clean it up with a scotch brite pad.
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  2. #2
    Hydrotoys's Avatar
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    Even the "good" piston is lean, and has some scrub marks.

  3. #3
    well I'm wondering why it has smash marks but the piston still looks whole? almost like something found it's way up there and not related to lean ? however it def wasn't running rich

  4. #4
    Moderator OsideBill's Avatar
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    Quote Originally Posted by ZMANN View Post
    well I'm wondering why it has smash marks but the piston still looks whole? almost like something found it's way up there and not related to lean ? however it def wasn't running rich
    That is classic detonation, nothing smashed just the result of thousands of tiny explosions.

    Mrbtd sounds like the classic midrange lean spot what was were you're carbs set to?

  5. #5
    Duke's Avatar
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    Quote Originally Posted by ZMANN View Post
    well I'm wondering why it has smash marks but the piston still looks whole? almost like something found it's way up there and not related to lean ? however it def wasn't running rich
    I agree. Something found its way in there, no sign of deto and can see some wash over all the dents. Its on the lean side but thats not your problem.

    That sucks!! At least it waited till late Saturday.

  6. #6
    Ya Boy Trevor bajaeater's Avatar
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    its weird how its only on the outer edges. I wonder what was in there to do that. definently lean.

  7. #7
    BoostCrzy's Avatar
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    looks like it ingested something...the edges are beat because the clearance is tighter on the edges of the combustion chamber....find any thing in your exhaust pipe?...

    heres a pic of a lean failure that resulted in pieces of material beating up the insides....not the same failure if you ask me...
    Last edited by BoostCrzy; 06-17-2008 at 08:55 PM.

  8. #8
    [quote=mrbtd;682288]

    Did the lean spot in the carbs get me ? If so why only on #1 wash for #2 is not bad.

    quote] I don't see any wash,,,,

  9. #9
    This is an interesting thread to me.

    I'm trying to learn how this stuff works internally but I am getting confused. There are so many "phrases" being mentioned here that I don't understand: "Load up", "Pistons were coated", "Lean spot in the carb", "Wash", "Scrub marks", etc.

    Then what really made me wonder was one guy said the cause of this failure was "definately detonation" while others said it was caused by junk in the cylinder.

    So, what is the cause?

    I'm in the process of rebuilding my top end and my pistons heads look like picture #2. You all say it's running lean. But, what exactly does that mean. Not enough fuel? Not enough oil? ????? And, more importantly, what should my pistons look like and what adjustments do I need to make.

    Finally, I'd like to see someone make a "sticky" about this subject - it would be extremely useful to a lot of people. Have pictures, discussions & detailed solutions for causes of piston failure.

  10. #10
    Hydrotoys's Avatar
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    Welcome to the GH forums bbealing. You have landed in the right place to learn these terms...

    What you are seeing are many experienced engine builders giving opinions on a diagnosis, with only limited information to work with.

    As he tears down that cylinder, (and I would do the other one as well for inspection,) we will see more info that will help him/us diagnose it.

    If possible, start your stay here by using the search function up top and keep it very simple.

    Start with the terms you have mentioned. Tons of info there, with pics, more than likely. Also check out something very important with 2strokes called "pressure testing." It's not what you think.

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