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  1. #1
    PWCToday Newbie
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    JS550 Piston Damage

    Good evening, friends.

    I tested my JS550's compression and got 120 in the PTO cylinder and 95 on the mag side. When I pulled the head, I noticed a hole that appears to have blown through the intake side of the edge of the piston (where the ring gap is positioned, if I'm not mistaken). You can see it at the bottom edge of the photo below.

    20170914_181417.jpg

    There is also a groove in the top of the cylinder wall in the same spot. I couldn't get a good picture; I'll try again tomorrow in better light.

    So, this is pretty obviously the cause of my low compression, right? What I'm wondering is, what could have caused this? I'm planning on replacing the piston and sleeve in the front, and doing a new ring in the back (that piston looks good). I don't want this to happen to the new one next season.

    I appreciate any and all ideas.

    Steve
    1987 JS550

    Riding Lake Erie around Madison, OH

  2. #2
    Frequent Poster
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    Oct 2013
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    Ohio
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    296

    Re: JS550 Piston Damage

    From the looks of the piston, I have to assume that the head has a lot of matching dents. Something has been sucked into the engine or come loose from inside it. The consistent size and shape of the dents, and the concentration of dents inside the squish band indicated it perhaps was a small pin from someplace. Perhaps one of the pins that keeps the rings in place?

    Good Luck,

    John
    2, 1998 TS 770 L, restored and running great. (Except for the one with engine that SBT put the WRONG crankshaft bearing into and is now sitting as I collect repair parts)

  3. #3
    PWCToday Newbie
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    Re: JS550 Piston Damage

    The head does have matching damage. The left side of the picture is the front side of the head.

    20170914_181659.jpg
    1987 JS550

    Riding Lake Erie around Madison, OH

  4. #4
    PWCToday Regular jakber's Avatar
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    texas
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    Re: JS550 Piston Damage

    Possibly carbon buildup on a port falling into the engine causing damage.
    ultra 150 lake travis texas

  5. #5
    PWCToday.com Is My Home Away From Home Chester's Avatar
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    Re: JS550 Piston Damage

    Pull the cylinders off and look at the crank and rods.

    Chester
    Speed is very expensive. How fast do you want to spend?

    Stand up:
    98 SXi-Pro

    Couches:

    96 XP
    86 X2

  6. #6
    PWCToday Newbie
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    Re: JS550 Piston Damage

    The whole motor is coming apart soon. I might have some time to tear into it this weekend, but probably not. I'll put up pictures as soon as I can. Thanks, guys.
    1987 JS550

    Riding Lake Erie around Madison, OH

  7. #7
    PWCToday.com Is My Home Away From Home hyosung's Avatar
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    Re: JS550 Piston Damage

    Quote Originally Posted by Chester View Post
    Pull the cylinders off and look at the crank and rods.

    Chester
    +1 , they look like the type of damage a needle bearing does when it tries to escape,
    2 x 650 sx custom build (my toys) http://www.pwctoday.com/showthread.php?t=425767

    modded X2 650

    stock looking ZXI 900 ""being cannibalized to sponsor a 650 sx build """

    Jerry Rigging is my specialty .....


    Originally Posted by hyosung
    so when you say i """ hammer and slap things together """ this may be so , BUT they are hammered and slapped together SOLID

  8. #8
    PWCToday Newbie
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    Re: JS550 Piston Damage

    The plot thickens. I was told by the previous owner that this ski had the piston port motor that originally came with it (it's a 1987). Well, when I took the intake manifold off today, there were reed valves on it (!!!). They were mismatched. The valve on the PTO side had three separate petals each on the top and bottom, and the Mag side has one solid petal on the top and on bottom.

    Can mismatched reed valves cause an overheating condition in one cylinder? The ski has one SBN44, if that makes a difference.
    1987 JS550

    Riding Lake Erie around Madison, OH

  9. #9
    PWCToday Newbie
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    Re: JS550 Piston Damage

    Here is the piston. I just got it out last night. I found that when I lifted the cylinder block off of it, the ring was facing almost 180 degrees off location, with the gap on the exhaust side. Since there's a little piece of metal for the ring to seat against, I'm guessing that could have held the ring away from the piston and caused my problem? The needle bearings were all in place and in good condition, Hyosung. Unless there's a roller in the crankcase that could have lost one...
    20170920_191120.jpg
    20170920_191422.jpg
    1987 JS550

    Riding Lake Erie around Madison, OH

  10. #10
    PWCToday.com Is My Home Away From Home
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    Re: JS550 Piston Damage

    That little piece of metal is supposed to keep the ring from turning and the ring ends should meet where that pin is. If the engine was assembled with the ring spun around it would have self-destructed rather quickly. Did a ring end break off and cause the damage we see? With the ring ends by the exhaust port it would have caught on the port. Was the other piston installed correctly?
    Riding mostly Lake Austin

    1984 JS440
    1989 650sx
    1991 X2
    1992 750sx
    1995 900zxi (qty 2)
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    Quote Originally Posted by cujo View Post
    God intended PWC to be two strokes, as well as dirt bikes, snowmobiles, and chain saws....

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