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  1. #1

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    "Subtle" blown piston

    Had my first engine failure last week, was expecting something more dramatic. Pulling kids on a double tube with my 223 hour '98 XL1200 and it started to die on me. (Use the ski an hour in morning and an hour in afternoon to pull tube) Revs cutting out like it was running out of gas but it wasn't. Unhooked tube and feathered throttle to run a little then it died on me and acted like battery dead when tried to start. Waited a few minutes then it fired and rode back to dock at about 40 mph max. Checked plugs, #3 black, electrode smashed down. Towed to ramp at 5 mph and trailered it. Took head off yesterday and here's what I found.



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    I run Yamalube at 40:1 premix. Wash on other pistons looks good. Rebuilt carbs with Mikuni kits over winter. Found an SBT tag on end of head cover, SBT shows shipped 8-9-06. Good pistons stamped at 1 mm over. Hope to have time this evening to pull cylinder body and will add more pic's. Does head need to be replaced? If anyone has any observations already it would be greatly appreciated.
    Thanks.
    Last edited by toyterrific; 07-25-2011 at 08:49 AM. Reason: added info


  2. #2

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    Quote Originally Posted by toyterrific View Post
    ........Checked plugs, #3 black, electrode smashed down......
    [Thanks.

    Sometimes this is an indication that one of your crank bearings let go. When you pull the cylinder, check for sloppyness on your crank.

    As far as your head is concerned, you may get someone to mill it down and reshape the dome ... get you 135 to 150 psig. It is kind of hard to see the damage on the dome, so it may not be quite so bad. If you do have it milled, the engine will be a whole lot snappier, but will run on 91-93 octane. Of course if you do this, you will have to rejet the carbs.

  3. #3
    Yamaha artisan Cutlass's Avatar
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    looks like it melted a little bit on the intake side.
    When I hear blown cylinder 3, first thing I think of is leaky rear crank seal causing a lean condition on number 3. Might be something to check. Either way, with that SBT tag on it, you might wanna tear the whole thing down and inspect EVERYTHING.

  4. #4

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    Finally got cylinder body off, here's what I found.
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    Guessing that ring spun, hung up on intake port taking out that chunk of sleeve?
    Piston gauge does not line up with damaged port & will not slide back and forth on pin as front two. Lower bearing not loose.
    Welding done on crank near all 3 rods. Does that mean I have a "trued & welded crank", an SBT repaired crank, or both?
    Do not have pool of oil in bottom of ski, making me think it is not a rear seal.
    Can any machine shop replace sleeve or am I better off buying a used cylinder body?
    Planning on leaving cases together in ski, flooding with brake cleaner and vacuum out about a dozen times to clean up any metal. Replace pistons, re-sleeve or replace cylinders and ride.
    Any thoughts?

    Thanks.

  5. #5
    I like Boobs. wetwolf's Avatar
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    With that kind of damage, I would pull the crank so that all the bearings can be properly checked. No sense putting a fresh top end onto an unknown bottom end.

  6. #6
    Yamaha artisan Cutlass's Avatar
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    Quote Originally Posted by toyterrific View Post
    Welding done on crank near all 3 rods. Does that mean I have a "trued & welded crank", an SBT repaired crank, or both?
    Take this with a grain of salt since I really don't know...but I've heard SBT welds their Chinese cranks so they stay in phase because the press fit is hit or miss.

    Quote Originally Posted by wetwolf View Post
    With that kind of damage, I would pull the crank so that all the bearings can be properly checked. No sense putting a fresh top end onto an unknown bottom end.
    +1...Absolutely have to pull the crank and properly inspect and clean with that kind of damage.

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