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  1. #1
    krispaintballz's Avatar
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    Compression Problems Gp1200

    I have a 98 65u motor that has been heavily modified then switched over to run 93 pump gas. I have a riva girdled head with 34cc domes. I just put in new wiseco pistons and tested the compression (Thanks to wonderful forums on "the right way to test compression" SBT). My results were 120-125-130 3-2-1. Why is the compression reading so low? I was thinking that the everything needs time to seat.
    I am also going to change all the waterlines on my triple pipes. Would it be ok to change them all to the same diameter? Factory said that there is 3/8 lines and 1/4 inch lines and i should keep that way. Thats seems too small to me. need the help. THANKS!




    1998 GP1200:
    Factory Pipe-Triple Exhaust
    Riva big bore kit
    12 Vane “D-75” Magnum Pump custom 14.20 pitch swirl prop
    Riva - Billet Girdled Head
    Riva - Flywheel
    Riva - 34cc Domes
    Riva - Porting & Polishing
    Buckshot 46mm Carbs
    Wiseco - Pistons
    Riva-High flow Flame Arrestors
    Riva-High flow intake
    Riva-Reed space/Booster
    Boyesen - Pro Series Reeds
    Riva/Boyesen-Reed stuffers
    Riva-Quick-turn steering nozzle
    Skat Trak-pump cone
    Riva-Triple intake fuel pick up
    Triple fuel lines and fuel filter routed directly to carbs
    Tiny Tach-External digital tach
    R & D–Re programmed Ignition
    Worx-Pro-Series Sponsons
    R & D–Ride Plate
    R & D–Aqua Vein Intake
    Riva-Dual Cooling System
    Riva-Speed Shop Work (welded crank, polished ports, etc.)
    Hydro Turf - Competition Mats
    Riva- Billet trim tabs


  2. #2
    WATER WOODY's Avatar
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    What brand of compression guage are you using? They will vary quit a bit between a cheaper one and a Snap-On.

  3. #3
    krispaintballz's Avatar
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    Snap-on world headquarters is 4 blocks from my house! My neighbor works there. Thats all we use around here. Even that would only make a small psi difference. I thought i would be sitting around 150 to 160lbs at least! I used the 1/2" adapter, Thats what yamaha said to use. 3/4" will give you wrong read. Thats what they said!
    Last edited by krispaintballz; 10-25-2007 at 08:16 PM. Reason: added some info

  4. #4
    You got a nice deal on that ski on Ebay . I have that same set-up and I have 41cc domes on that riva head. I think you might have the wrong domes?

  5. #5
    WATER WOODY's Avatar
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    Ive seen 8-10 psi difference in guages before.

  6. #6
    krispaintballz's Avatar
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    Quote Originally Posted by Metal4130 View Post
    You got a nice deal on that ski on Ebay . I have that same set-up and I have 41cc domes on that riva head. I think you might have the wrong domes?

    That guy that i got it from was a really nice guy! I runs really good. If i put those 41cc domes in that would lower the compression more.

  7. #7
    krispaintballz's Avatar
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    Quote Originally Posted by WATER WOODY View Post
    Ive seen 8-10 psi difference in guages before.
    Yeah but with 34s you would think i would have a higher compression.

  8. #8
    WATER WOODY's Avatar
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    Whats the compression supposed to be,150-160?.Excuse me Kris...WELCOME to the forum...

  9. #9
    krispaintballz's Avatar
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    Quote Originally Posted by WATER WOODY View Post
    Whats the compression supposed to be,150-160?.Excuse me Kris...WELCOME to the forum...

    I don't know! I just got it and found that the compression was a little low with 34cc domes i thought. With the 34s i would have thought 150 -160. Come on 120-130 psi after all the mods, doesnt that seem a little low. I am guessing someone ported the s***t out of it. what do you think?

    Oh thanks for the warm welcome, finnally somewhere i can work out my problems without paying the dealers big money and find out the guy that worked on it knows less than you (meaning us, the public) do! Sorry it is the truth sometimes.
    Last edited by krispaintballz; 10-25-2007 at 10:26 PM.

  10. #10
    cheatin' piston popper addicted's Avatar
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    how big of a bore?
    my ported 65U is 85mm and with 37cc domes i get 150psi. If your battery is low, you comp will be low, and if the exhaust port is super high it will lower it as well.

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