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  1. #1

    Hole in piston but why!

    Hi
    my polaris sl 750 just burned a nice hole in my piston (pto - nearest the end of the ski right) CRAP!
    I just rebuild the carbs with mikuni parts and adjusted them very carefully, rebuild my pentagon 3 outlet fuel pump and also fitted a brand new fuel filter.. 5 days ago i tested it and it ran perfekt, after 2 hours i went home. Then today i went out again, fired up good, but wouldn't take full rpm? 5 minutes later - hole in piston!
    What went wrong, i have only filled up gas since the last ride! can a blocked fuel filter be it? Because some how it ran lean and just 5 days ago i ran it very hard for 2 hours with no problems..

    Morten


  2. #2
    Running premix and brand new fuellines..

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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  4. #4
    My guess would be an air leak caused by a faulty gasket after reading our link.. The ski has been standing still for about 6 months, then last week i drove it varm (not hot, but warm) then it cooled down and yesterday warm again maybe that caused a weak gasket to fail?
    Think I will fit new headgaskets, carb gaskets, intake manifold gaskets.. don't know about the exhaust gaskets...
    If you wanna set the carbs a little bit richer which way should you turn the screws in or out? I have an outboard mercury engine and to make it run richer you have to screw the adjustment screws IN.. and my logik was more fuel, more open screw (:

  5. #5
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    My guess is a faulty crank seal before any off the other seals. Do a leak down test.

    Screws OUT for more fuel. IN, shuts off the supply of fuel.

  6. #6
    BlueFishCrisis's Avatar
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    Quote Originally Posted by Morten1988 View Post
    My guess would be an air leak caused by a faulty gasket after reading our link.. The ski has been standing still for about 6 months, then last week i drove it varm (not hot, but warm) then it cooled down and yesterday warm again maybe that caused a weak gasket to fail?
    Think I will fit new headgaskets, carb gaskets, intake manifold gaskets.. don't know about the exhaust gaskets...
    If you wanna set the carbs a little bit richer which way should you turn the screws in or out? I have an outboard mercury engine and to make it run richer you have to screw the adjustment screws IN.. and my logik was more fuel, more open screw (:
    Yes, opening the screw will allow more fuel in. However, Xlint's comment about leakdown testing does not only apply to head gaskets, carb gaskets, and intake manifold gaskets. More often than not, a failed leakdown is due to an aging, worn, or cracked crank seal. Sealing off all of the openings and pressurizing your motor will reveal the source of the leak.....

    You may also have a crank index issue, which can cause a lean condition as well.

    what did your piston wash look like??

    I have spare single gaskets available here including crank seals should you need them. PM me if you are interested....

  7. #7
    bowsniper's Avatar
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    Dont use that mercury ouutboard advice to a blue fugi polaris engine.its the other way around.OPENING. the high speed or low speed adjusters. Makes it richer.

    Whats the compression before the burndown? What rato 2-stroke were you running??

    Do you mean you have a Pentagon pump? The original pump for that ski is a single outlet and guys upgrade to a triple oulet round type like the original but it uses 3 outlets.

    If your using a pentagon pump,thats fine too. I was jusr curious as to exactly which one.

    So you were running an untouched engine? But with a pent pump correct?

    If so......how did you route it? Guys on here follow BEERDARTS fuel routing diagram and it works perfect. Did you hse his routing?

    Is your restrictor in the fuel return hose? Maybe its clogged. Or a diaphram got a hole. Or sticking. Case leak is a possibitly.

    Check the crank seals for protruding out on 1the side

    Where did you mount the pent pump? Is it above the pulse line? That line needs to be 12 inches or less.


    Check the base nuts on the jugs ro .see if they are still torqued. Even the case could be leaking.

    Did you take off the blue caps to a adjust the carb needles?



    Did you set all the carbs by the book? Each one is different.
    A needle or seat could be stuck or clogged too.that could cause a lean condition in 1the hole.
    What do the piston tops look like? All black or just the pto.

  8. #8
    Thanks for responding!

    I found a complete gasket set including crank seals i will order.
    Don't know what the compression was before it broke down.
    Use the same 2 stroke oil that i use for my boat, the only thing i could get in my local marine store.
    Yes i have the 3 outlet pentagon fuel pump and I just rebuild it a month ago and it is mounted just under the flame restrictor, so about same hight as the top of the carbs.
    Not sure it is an untouched engine, think the previous owner said something about there been change a piston but not sure at all and had to change a lot of things he had made wrong.
    I did Beerdarts fuel routing and just mounted a fresh fuel restrictor a month ago + a new fuelfilter.
    The carbs has been very carefully adjusted with the right specifikations an the blue caps of and rebuild.
    Don't know how the other pistons look, just confirmed no compression on PTO because of a hole in the piston.

    Think it's going to be a complete gasket replacement, hone the cylinder and fit a new piston. Crap! Do not feel like taking the engine out at all!
    Mesured the cylinder and it was 69,75mm so that is just a standard piston right? On ebay they want 100$ for a standard piston and about 70-80$ for a 0,25/0,50 oversize piston is that right? Maybe different quality??

  9. #9
    BlueFishCrisis's Avatar
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    http://www.ebay.com/itm/200407035965...84.m1423.l2649

    I have changed out two pistons / cylinders without removing the engine. It is possible...... Although I have SLTs and can climb into the hull to work on them (well almost)

  10. #10
    I have removed the cylinder without removing the engine so changing the piston shouldn't be a problem, it is all gaskets i'm worried about. Pretty sure i have to take out the engine to change crank seals etc.

    The link you posted says: "Standard size is 69.75mm, Sizes available: .25(.010), .50(.020),.75(.030), 1mm(.040)" does that mean you can't get a standard size or??

    I was looking at this one and it seems to be a standard size 69,75
    http://cgi.ebay.com/ws/eBayISAPI.dll...m=330756800927

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