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Thread: hole in piston

  1. #1

    hole in piston

    I have a 1994 sl 750 for the second time now it has burned a hole in the pto side piston not for sure why hoping someone can give me a starting point to look into thanks


  2. #2
    Connecticut CrazyA's Avatar
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    This was put together by Kevin and is an excellent starting point for the Fuji's when having trouble like this...

    http://www.greenhulk.net/forums/showthread.php?t=82682

    And welcome to the forum!!!!

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Welcome to the Hulk Dooman.

    That list I wrote up, which CrazyA linked for you, covers most of the basics.

    I'd recommend trying a leak down test first.

    Make sure the fuel restrictor is in place. Inside the return fuel line about 3/4" past the the last carb.

    Thoroughly inspect the carbs. There's a filter on the inside that can get clogged.

    Clean and inspect the fuel shut off valve. (petcock)

    Inspect or replace the fuel hoses, even the ones inside the fuel tank if so equipped.

    And finally, replace the fuel pump with a triple outlet pump. John Zigler sells these for about $26.

  4. #4
    thanks for the info I will try all of this I did run a leak test last summer and it checked out allright. Is the fuel pump an a problem with this ski I have read alot about changing this although we have ran it a long time before this happened know it just dosent want to stop. P.S. this is the reason I went away from thier snow machines. thanks

  5. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by dooman View Post
    ...Is the fuel pump an a problem with this ski I have read a lot about changing this although we have ran it a long time before this happened know it just dosn't want to stop...
    The stock 650 & 750 fuel pumps slowly lose their ability as they age to provide full fuel pressure at full throttle. The triple outlet fuel pump has extra flow capacity, so even as it ages it is still strong enough to keep the carbs happy at WOT.

    That said, the more common immediate cause of holed pistons is clogged or gummed up carbs. This is probably the number one thing that kills these engines.

    The fuel lines get old, and sometimes the fuel shutoff valve develops air leaks.

    So you change the fuel lines and fuel pump to be sure they are NOT causing trouble.

    You clean/rebuild/lube or replace the fuel valve to be sure it is not leaking air into the fuel lines at speed.

    And you rebuild the carbs because all of the above doesn't help if the carbs are dirty.

  6. #6
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    The single outlet pump was barely able to supply enough fuel when in great shape for a 3 cyl. As it wears it will starve the engine of enough fuel to create lean conditions.
    Most here will recommend installing a triple outlet pump. Most would recommend john zigler for a new pump. Very reasonable on price too.
    The fuel routing schematic can be found here.

  7. #7
    Connecticut CrazyA's Avatar
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    The on/off valve too

    The fuel shutoff valve should be looked at real well too.

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    If you feel the engine is sealed well, then address the ignition timing and fuel system.

    Do a crank indexing test. That'll tell if your crank is out of phase.

    New hoses will run around $20

    A new fuel pump about $26.

    The reason the triple outlet is recommended, is because the single outlet would feed one carb, then the next, and the next. By the time it gets gas to the last one (MAG) there isn't enough to supply it's needs. That's why the MAG would burn down. If the restrictor isn't installed, there isn't enough fuel pressure supplied to the PTO (last) cylinder, which forces fuel into the fuel chamber.

    Rebuild the carbs. Kits (Mikuni super BN 38/44mm) can be bought for around $35 ea. Use only Mikuni OEM parts when doing this. OR you can buy the parts individually since the kit contains many pieces you won't need. NOTE: that the needles and seats (2.0) are not included in the kits. Those run for around $15 ea

    For around $150, you can rebuild your entire fuel system. It may solve your burn down issue, or eliminate it from the possible cause.

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