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  1. #1
    88kawi5fiddy's Avatar
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    gpr compression after rebuild

    I just finished rebuilding my GPR and broke it in with 15 minutes warm up easy running, 15 minutes medium throttle use and 15 minutes short 3-5 second wide open runs with cooldowns between everything. when I got home I checked compression and all three cylinders are 100 102 105 psi.
    Do the rings need more time to seal the cylinder? Plugs look nearly perfect.
    The head was milled 0.020 and squish is 0.055.


  2. #2
    Prism's Avatar
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    It's been (depending on factors) my experience that compression usually peaks between 4 and 10 hrs of use after a rebuild. A richer fuel/air ratio and premix takes longer (up to 10hrs) and leaner ratios with oil injection will be closer to 4 hrs use. I inspect with a bore scope as the wash colors the piston. I also pull the head at 10 hours to get a good visual of the cyl walls. If everything still looks great and the readings are still low I'd try another gauge. Make sure the engine is warm, battery charged and throttle is wide open when doing the comp test. From your measurements it looks like you can take a head gasket layer off. Your squish will be closer to 0.045 and compression should be about 145-150 (with stock port height) which is where you want to be. Don't go tighter than .035 squish unless you're sure about your measurements and you know the risks.
    Last edited by Prism; 03-23-2015 at 08:48 AM.

  3. #3
    Myself's Avatar
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    Looks pretty even, I would guess your gauge is reading low.

  4. #4
    88kawi5fiddy's Avatar
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    Okay. I got .020 milled off and am running two layers of head gasket. I read that you only want to take the "lip" off the head because any further will start to flatten the edges of the squish. Have to wait two weeks to get back to the lake. Thanks for the replies.

  5. #5
    Prism's Avatar
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    What year is the ski? Do you have a fuel controller or carb jetting mods? Just don't melt it down by using the stock a/f ratios.

  6. #6
    My experience is what compression you have after assembly (rebuild) is pretty close to what you will have after breaking. 1 or 2 psi gain. Maybe. Maybe your battery is not full charge or gauge is off. What was the compression before rebuild?

  7. #7
    Prism's Avatar
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    Just curious (because of a different thread) did you change only the rings during the rebuild or both the pistons and rings? If only the rings, how many hours total on the pistons? I might add break in completion won't give you a 30psi gain...maybe more like a 5psi increase.

  8. #8
    88kawi5fiddy's Avatar
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    carbs been modded with drilled returns and ferchos jetting. odd part is i "should" go to 122.5 mains. 120's have my HS at 2.5 turns out but it was also pretty cool that day. 3000ft at maybe 60 air temp.

    everything is new. new cylinders from a member on here and wiseco pistons. i didnt test till i got home and my experience with forged is it drops 10+ psi when they are cold so it is probably fine. ski starts and idles perfect. start button only has to be held 1/3 or 1/2 a second for the motor to fire.

    ill just retest it at the lake after another hour of break in when the pistons are hot. the rings probably havent even started to break in.

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