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  1. #1

    Join Date
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    97 GP 1200 is done

    My ski is together. Crankworks crank, fresh 85mm bores and Pro X slugs. everything is meticulously assembled. I am nervous to start it though. I bought an R&R Jettech head. Brand new and a super nice piece. However the domes are 39.5 cc. will this be ok on 93 octane. If not I can get 100 octane. My machinist told me that the overbore will slightly drop compression. Is it true? will this thing be OK?


  2. #2
    Hydrotoys's Avatar
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    Uh, no. An overbore (I assume with a larger piston) will INCREASE your compression.

    Do a compression check and see what it looks like.

  3. #3
    jonny5's Avatar
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    your goona be about 165psi to 175psi if your port hight is the same as stock..

    I assume no porting....???

    What ignition stock or riva cdi box.

    165 psi should be ok on 93 what is your squish???

  4. #4
    Flying Scotsman's Avatar
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    Good deall
    My guess would be 160 psi
    Do a compression check.
    Usualy no problem at 150-155 psi with 91 octane.
    Getting to be border line though so be carefull
    I have been running a Pro tec head with 44cc and a Pro design head with 42cc domes.
    With 85mm cylinders I was at 155psi.
    How soon till you hit the water.
    SCOTT.........

  5. #5
    Moderator RX951's Avatar
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    My machinist told me that the overbore will slightly drop compression.
    when he said overbore, is the tolerance set slightly loose to make room for expansion? ring end gap and piston


    break out the guage and get some baseline compression numbers

    85mm GP12, dude....that should make for one strong holeshot monster

  6. #6

    Join Date
    Dec 2005
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    Jax Fl
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    i will get a gauge on it saturday. And stock ignition and stock porting. I will check squish too.

    My mods are

    85mm Pro X pistons
    39.5cc Jettech Head
    Crankworks T&W crank
    Riva spark arrestors
    No chokes, Primers
    14/20 solas J
    R&D ride plate
    R&D intake grate
    Riva Trim Tabs

  7. #7

    Join Date
    Dec 2005
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    Beaufort, SC
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    119
    You might want to get an aftermarket tach too, if you haven't already. I'm real curious to see what rmp's you're gonna pull with that J prop.

    Keep us posted. I'm getting ready to split my cases this weekend and send the crank out. I've got everything else I need to put this thing back together, except a gasket and seal kit.

  8. #8
    Flying Scotsman's Avatar
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    You have to much prop with a J in my oppinion.
    Will bring rpm down and raise temp from loading up.
    If you were ported or piped then the 14/20 J
    13/19 should work better.
    SCOTT.........

  9. #9

    Join Date
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    think same on prop,have old riva cdi box if you what (purple one)
    bigger bore will raise comprestion(greater volume)have you jeted carbs?

    later cd

  10. #10

    Join Date
    Dec 2005
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    Stock jetting. I started it today just for a second or so. Sounds crisper than it did before.

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