200psi in js500!!
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  1. #1
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    200psi in js500!!

    hi guys, well 6 months ago I fully rebuilt my js550 new crank bearings / seals/ gaskets set / 1mm oversized pistons and rings, the full works, and she lasted 9 hours on the water before having a 4 corner seizer on the rear cylinder, damage was luckily able to be honed out and new pistons replaced, new seals again and gaskets, done a compression test and its 200/210psi no wonder she went bang before! however ive taken the head off and I'm thinking someone has taking loads off the face as I see replacements have a small step before it goes into the dome of the head where the plug sits, however mine doesn't have a step its flush and then goes straight into a dome which I'm assuming will cause less space for the air and fuel etc, pictures below show my cylinder head, could this be my issue,
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  2. #2
    PWCToday.com Is My Home Away From Home john zigler's Avatar
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    Re: 200psi in js500!!

    Your head has been cut. It appears your domes have been cut as well.

    200 PSI is not uncommon in a 550, and can be fine as long as the engine is built to handle it, and you are running race gas.

    A 4 corner seizure is typically from overheat, or improper piston to cyl clearance.
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  3. #3
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    Re: 200psi in js500!!

    No I was running normal unleaded fuel, and 2 stroke 42/1 mix, but I want it to run less so a standard head will sort this won't it?

  4. #4
    PWCToday.com Is My Home Away From Home Benflynn's Avatar
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    Re: 200psi in js500!!

    Get a copper head gasket, that will knock it down a bit

  5. #5
    PWCToday.com Is My Home Away From Home mcn6's Avatar
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    Re: 200psi in js500!!

    As already mentioned... Head has been milled and re-chambered. It is also pretty common on 550's to deck the cylinder to reduce squish clearance.



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  6. #6
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    Re: 200psi in js500!!

    I just want to run normal fuel and oil mix, rather than have something that's running high psi and needs special fuel to compensate this but basically it's a head swap isn't it

  7. #7
    PWCToday.com Is My Home Away From Home mcn6's Avatar
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    Re: 200psi in js500!!

    You can try a stock head and/or gasket or whatever, but without knowing if you cylinder has been decked it is hard to say for sure?

    Stock PP 550 cylinder height is 116mm? If you can either confirm this dimension (or tell me what your current cylinder casting height is) I could tell you what to do. Short of that, you are stuck with trying different head and gasket combinations until you get whatever cranking psi you are looking for.



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  8. #8
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    Re: 200psi in js500!!

    Ok that's great, I shall have a look when I'm back
    Last edited by Dutchyboyyyy; 07-26-2017 at 01:47 PM.

  9. #9
    PWCToday.com Is My Home Away From Home mcn6's Avatar
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    Re: 200psi in js500!!

    Cylinder casting height = Head gasket surface to base gasket surface.



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  10. #10
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    Re: 200psi in js500!!

    Super job Ill get back to you asap

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