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  1. #1
    silvercrxsir's Avatar
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    99 gp800 new top end with a problem

    I just installed a cylinder exchange kit from Sbt with new resleeved cylinders and wasbreaking it in exactly the way the instructions said and it started making noise checked compression and the front was 120psi and rear was 150psi,carbs were rebuilt with genuine mikuni kits pop off and needles are all set to factory settings.front piston and cyl is scored all the way around the cylinder and piston are coated in oil so it doesn't look like a lubrication issue top end has around 2 hrs on it plugs are a nice brown color any idea what could have caused this and what do I need to do to fix it.


  2. #2
    Yamaha artisan Cutlass's Avatar
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    post some pics of the piston and cylinder. There are usually some clues that can help us point out the problem.

  3. #3
    silvercrxsir's Avatar
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    It appears that the front crank seal might possibly be leaking and since I don't want to trash another piston I will change the seals.does anyone know what is involved in doing this and do I need to split the cases and how do I vaccum check it when done

  4. #4
    Yamaha artisan Cutlass's Avatar
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    Yes, split the case to do the seals and pressure test the engine when done. There are several threads around here with pressure test procedures and ideas on how to block off things. I cut up an old inner tube to block off the intake and exhaust and pressurized the engine through a pulse fitting.

  5. #5
    Buaidh No Bas howestek's Avatar
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    Did I misunderstand something? Did you say 150 psi on a stock 99 gp800? Wow! 120 psi sounds about right for the front cylinder. How accurate is that gauge?

  6. #6
    silvercrxsir's Avatar
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    The head was resurfaced .010" the front had 120 psi with major scoring

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