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  1. #1

    Gp1200r tuning complete

    It's taken 6 months of trial and error but yesterday finally had a breakthrough. I modded the ski back in July but used a wrong-cut head which bottomed out the compression. So I took the stock head and had it mildly cut locally for 135psi after removing the center layer of a brand new gasket.

    For the head; I had the squish recessed 40tho and the head milled back 50tho for 135psi. Before when stock the engine had perfect 100 across all three (so it's low but all even). I'm thinking when it's ring time compression may get up to 150.

    What a difference. The engine is pulling an extra 300rpm from before, coming from a head which was only giving 80psi. Lesson learned to not use a head designed for a 1500cc motor on stock bored.

    With modifications listed in sig, does 7290rpm finally sound right? The most I got last summer was 7000. Stock speedo said 63mph before now it's saying 66mph.

  2. #2
    OzarkYamMonster's Avatar
    Join Date
    Oct 2012
    Lake Of The Ozarks
    Great that you found & fix your head problem. The compression as you noted is still very low. However, 66 on that ski is great. Good Luck

  3. #3
    04gtxsc's Avatar
    Join Date
    Mar 2006
    Sound like you finally got it running right. About the PSI your gauges could be off. I have seen three sets of gauges on the same motor give three varying readings. The important thing is they are all the same and the ski seems to be running good. Run with it and have fun.

  4. #4
    Thanks guys. The ski has original block with 89 hours on it. When I got the ski at 55 hours the compression was already/always 100 on each. The cyl walls look great/perfect. Three gauges all the same. Now, I had my Snapon gauge calibrated also with a new shredder valve and here is all it's read:

    GP1200r shaved head-135psi
    X2-750 (JSU head) - 153psi (185psi)
    X2-650 stock - 155psi
    550SX - 150psi
    JS550 - 120psi

    This GPR just likes being a little low.

    The gauge is accurate. Now that it's running higher comp I'll monitor the compression for the next few months. I've already thought a re-ring job would probably do just the trick if nothing else gets replaced.

    Also, I never mess with the power valves when I do a compression test. I always just crank with WOT.

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