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  1. #1

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    Tiny hole in the Mag Piston

    I've done this dance before with my 95 SL750.

    It's been about 5-6 years since my last carb rebuild and I think I used after market carb rebuild kits then – my bad. Will be buying genuine Mikuni ones this time and replacing the fuel line again just to be safe.

    The hole is tiny and from what I can see with the head off, there's no damage or scoring on the cylinder wall. I'm planning to pull the engine and flush the lower end on all three cylinders to ensure there's no metal pieces remaining down there. I will index the crank and ensure the timing is positioned correctly. I'm planning to only replace the piston with a SBT piston kit and wrist pin bearing with a STD size since my cylinders have never been honed or bored.

    Is there anything else I should be replacing or doing while I'm doing this? This is likely the last major resurrection of my SL, and I'd rather not break the bank. No doubt I've seen more on repairs over the past 27 year. After this one I might let her float off to the glassy lake in the sky or wherever old watercraft go when they're done.


  2. #2
    Click avatar for tech links/info K447's Avatar
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    I forget exactly how the Fuji engine is configured but I would be thinking about replacing the crankshaft end seals if they are old.

    When was this engine last rebuilt, if ever?

    Thermostat and pressure bypass valve - check for wear, aged seals, etc.

  3. #3

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    Quote Originally Posted by K447 View Post
    I forget exactly how the Fuji engine is configured but I would be thinking about replacing the crankshaft end seals if they are old.
    The crank seals and all the bearings were replaced when I had my crank rebuilt two years ago due to a failed bearing.

    When was this engine last rebuilt, if ever?
    Never fully at one time. New PTO piston, rings, and cylinders twice in 2004 and 2010 I think, and the crank rebuilt like I said in 2019. What would a rebuild entail? New pistons, rings, wrist bearings, and cylinders? Something like this from SBT? This appears to be a more cost effective solution than boring existing cylinders and new oversized pistons.

    https://www.shopsbt.com/jetski/polar...1992-1995.html

    Or is there more I should consider doing?

    Thermostat and pressure bypass valve - check for wear, aged seals, etc.
    They seemed fine wear-wise last year when I had them apart with no debris.

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    My guess is just age of the rebuild kits. With this crap gas we have now, it does wonders on the rubber parts inside the fuel system.

    Take a look at the fuel selector valve too. That's got rubber parts and is very old as well.

    Crank seals would be a good thing to inspect, but if you just did a crank rebuild a couple years ago, should be fine. A leak down test can tell you if you need to address the seals.

  5. #5

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    Quote Originally Posted by xlint89 View Post
    Take a look at the fuel selector valve too. That's got rubber parts and is very old as well.
    Thanks, will test the fuel selector/shutoff as well, but it was replaced about 5 years ago with one from Watcon.com, so I expect (or hope!) it's still good.

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    You're prob good to go if it's only 5 years old.

  7. #7

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    So…

    Rebuilt the top end. New cylinders, pistons, rings, and bearings.

    Rebuilt the carbs. Yes it was an SBT and not a mikuni kit?!

    New fuel lines.

    New oil lines.

    New spark plugs.

    Broke it in properly over a tank of gas. Plug chops looked okay. Piston wash looked overly rich on all three cylinders. Checked them last weekend after 4 hours of use.

    Then today after about 8 hours of total use, I burnt a hole on the MAG piston again! Grrrrr!

    Either I didn’t tune the carbs properly, the SBT carb kits let me down, or there’s something wrong with my MAG carb, since that was the same one that holed last time. So pissed right now.

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Do you have the restrictor in the return hose?

    Did you ever do a leak down test?

  9. #9

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    Tiny hole in the Mag Piston

    Did the leak down. Took about 3 tests but ended up with 10 psi for 3 hours.

    Restrictor is in and working.

  10. #10
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Leak down sounds like it should be fine then.

    Restrictor is working? Have you tested fuel pressure to state that? If so, what is your readings at idle and at 3,000 RPM?

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