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  1. #1

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    Piston replaced, more SLT 750 problems :(

    hey yall, replaced my holed piston in my 750 with the wrong size, so got the right one in (thanks aaron!) and headed to the lake. it starts up well and idles smooth but lacks any real power. I varied the RPMs as I have heard that is ideal for a new piston/rings. down low it didnt have the punch it used to, and topped out at 4100 rpm and 28mph. naturally i didnt run wot for long as I was trying to vary rpms but that was all i could get out of it. I verified that all lines were full of fuel, all the air was burped out and fuel was flowing. i will pull the head soon to check wash on the new piston. i'll check the plugs too. any ideas as to what the problem may be? history on the boat: had it a year, put in triple outlet fuel pump, new fuel filters and lines, replaced holed piston. thats all i've done with it.


  2. #2
    Water Bum Rodneyae's Avatar
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    Check the crank shaft index.

  3. #3

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    Which cylinder went down last time. What were the plugs like or the was on the piston now. Any photos available?

  4. #4
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by 4stroker View Post
    Any photos available?
    There are several reasons for a failure like this, here are the most common; air-leak at the magneto side crankshaft seal, too lean carb jetting, too far advanced ignition timing or faulty igniter box, too hot of a spark plug range, too high of a compression ratio, too low octane fuel.
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  5. #5
    Connecticut CrazyA's Avatar
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    Quote Originally Posted by Fish Man View Post
    hey yall, replaced my holed piston in my 750 with the wrong size, so got the right one in (thanks aaron!) and headed to the lake. it starts up well and idles smooth but lacks any real power. I varied the RPMs as I have heard that is ideal for a new piston/rings. down low it didnt have the punch it used to, and topped out at 4100 rpm and 28mph. naturally i didnt run wot for long as I was trying to vary rpms but that was all i could get out of it. I verified that all lines were full of fuel, all the air was burped out and fuel was flowing. i will pull the head soon to check wash on the new piston. i'll check the plugs too. any ideas as to what the problem may be? history on the boat: had it a year, put in triple outlet fuel pump, new fuel filters and lines, replaced holed piston. thats all i've done with it.
    Did you ever figure out the initial cause of the failure?

    You tell us what happened.

    Was everything fine and it just "happened"?
    Did you ever hydrolock it?
    What spark plugs were in it?
    Bad gas?
    Air leak somewhere?
    Any mods done to it?

    Wish we had pics of when you had it apart.

    Need for info on original failure.


    .

  6. #6
    xplayer2885's Avatar
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    Once you pull the domes, you can start by proccess of elimination. If the piston wash is on the rich side, the hi speeds are to rich, if the MAG piston wash is looki rich you can eliminate a leaky crank seal. Next would be to check the indexing. Thats what i did. Its a PITA but woth it in the end.

  7. #7
    xplayer2885's Avatar
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    Quote Originally Posted by xplayer2885 View Post
    Once you pull the domes, you can start by proccess of elimination. If the piston wash is on the rich side, the hi speeds are to rich, if the MAG piston wash is looki rich you can eliminate a leaky crank seal. Next would be to check the indexing. Thats what i did. Its a PITA but woth it in the end.
    An also if the rpms run away on the trailor, meaning they keep climing, this is a strong signal of a leaky crank seal. If the rpms arent runnign away from you, i wouldnt worry too much about the seals. But then again you can run a leak down test.

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    4100 RPM and 28 MPH sounds like limp mode to me. Did you hook up all wiring correctly? Low oil, low fuel, or overheat will limit RPM to around 4200 RPM. Is your MFD working properly?

  9. #9
    Connecticut CrazyA's Avatar
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    From Kevin Cameron.....



    .
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  10. #10
    ph2ocraft's Avatar
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    Quote Originally Posted by CrazyA View Post
    From Kevin Cameron.....



    .
    Well, I can't seem to quote the article.
    On a Fuji engine the 7's were updated to the 8's as a cooler running plug BUT understand a 7 still needs a reason to burn a hole in ONE piston and not the others, my guess is the article was written for a thumper and not watercraft specific??

    Either way for the 7 to burn a hole in the piston of just one cylinder and leave the others in tact (assumption on my part), something would have to be affecting that one particular cylinder.

    Is it running that much hotter?
    Is the timing in that one cylinder to advanced?
    Is there a lack of fuel to that cylinder?

    I'd want to make sure your not in limp mode as 89 suggested.
    How's the crank index?

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