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Thread: Cracked Piston

  1. #1
    JET250's Avatar
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    Cracked Piston

    it was running sweet until the piston failed.



  2. #2
    Hmmm it didnt sound right. You could almost here it pinking the piston.

    What were your fuel settings and what was the RPM verses speed.

  3. #3
    Take the time to smile sirbreaksalot's Avatar
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    Quote Originally Posted by Skips313 View Post
    Hmmm it didnt sound right. You could almost here it pinking the piston.

    What were your fuel settings and what was the RPM verses speed.
    11.7 AFR, 50psi fuel 8300 didnt have gps during tuning, but later that day fullish 3/4 tank ,sitting up on seat, riding up 4-5km current , We 72 at will @ 8250
    seat was off in vid, and full riva box & free flow ex
    During the tuning it was running great no dramas, wasn't till next day it took a crap

  4. #4
    50psi?

    Not a Walbro and if it is what one?

    I run fat at 50 PSI.

    Maybe what Im hearing is just the video.

    8300 RPM must be a prop thing. I dont run those RPMs with stock prop unless we have a piston kit.

    Is this the piston that cracked?

  5. #5
    Take the time to smile sirbreaksalot's Avatar
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    Quote Originally Posted by Skips313 View Post
    50psi?

    Not a Walbro and if it is what one?

    I run fat at 50 PSI.

    Maybe what Im hearing is just the video.

    8300 RPM must be a prop thing. I dont run those RPMs with stock prop unless we have a piston kit.

    Is this the piston that cracked?
    yep, left it slightly fat , as dave does heap of WOT riding, target F/pres was 46-47 gave me 11.7-8
    50 psi was idling, It did drop to 46ish at wot
    GS 342 walbro, external reg
    Timing was abit high for safety 10 deg, will be dropped to 6 later

    (this set up is the Copy of J Wilson style, but I just revisiting ideas)
    Let me also add that after speaking to some very informed people in the KAWI race scene, This is nothing more than bad luck the piston cracked, Not the first time it happened on 07 boats, atleast the260x has thicker crowns to avoid this prob

  6. #6
    Your on the money then.

    Is that AFR at WOT? What is your AFR at idle with psi at 50.
    Did you try to drop the idle psi to 46 and add the power shot to add fuel in mid and top. That way it will clean up the bottom for you.

    Wait a minute. What injectors? Stock, then fine but Pink and your FAT>>>

  7. #7
    Take the time to smile sirbreaksalot's Avatar
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    Quote Originally Posted by Skips313 View Post
    Your on the money then.

    Is that AFR at WOT? What is your AFR at idle with psi at 50.
    Did you try to drop the idle psi to 46 and add the power shot to add fuel in mid and top. That way it will clean up the bottom for you.

    Wait a minute. What injectors? Stock, then fine but Pink and your FAT>>>
    Now after all the topend was tuned ,yes Afr was at WOT, I was going to connect the vacum line to the reg and see if that leans the bottom end out making it crisper.not that it is bad the way it is


    Stock injectors

  8. #8
    Is that piston with the crack straight from the boat. It has a great burn pattern but might show signs of washing.

    Any more info you can give about the set up.

    The only time I cracked a piston like that was due to water droplets getting into the cylinder. That water would explode and knock on the piston untill it crack. It had some funny washing spots on the piston. Some parts where black and others were cleaned off.

    Boost controlled regulator? 1-1 on regulator would put it from 50 add 13psi for pulleyed charger would be around 63psi? We dont use that because the roots is instant boost. So it would see a running addition to fuel pressure instead it on or off. ECU bleads the fuel depending on the manifold pressure.
    Now youll loose some of that due to boost restriction. Depends on the injectors ability to spray under boost resistance but surely they will be fighting the boost.

    Not challenging you at all, Im just throughing what Ive found out there. Im in this to learn and hope other s do the same with the info.

    Shoot what ya say we start another thread and move this stuff over to it. Maybe call it "Piston cracking" "250X Piston Cracking"? IDK.

  9. #9
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  10. #10
    Done I'm going to bed now. Catch this tomorrow. Good stuff.

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