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  1. #1

    1999 GP 1200 With Low Compression on 3rd cylinder BUT runs fine (57mph - rough water)

    All,

    I am wondering if it might be time to rebuild this engine. It only has 108 hours on it and seems to run fine on trailer as well as in the water. I just took it out and was doing 57mph in rough water. I did a compression check recently and got 115-120 on the first two cylinders and then got around 70 on the 3rd cylinder. Everything is stock on this ski, and it has been sitting for the past 2 years. (I just purchased it from someone else). I put some 2 stroke oil down into the 3rd cylinder and the compression shot up to 90. Any suggestions here? Can I keep running it the way it is, or will this tear up the motor? Should I rebuild all three cylinders with a top end kit, or just that cylinder. I just don't understand why it is running so well in the water with the 3rd cylinder having that low of compression.

    Compression test was done with a cold motor and with an OK gauge, but not the greatest. Did not do WOT.

    Thanks for any assitance you guys can provide


  2. #2
    why is it that low ? would be my first concern, find the problem that caused the low compression, then fix the comp issue.

  3. #3

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    Quote Originally Posted by cmone42 View Post
    All,

    I am wondering if it might be time to rebuild this engine. It only has 108 hours on it and seems to run fine on trailer as well as in the water. I just took it out and was doing 57mph in rough water. I did a compression check recently and got 115-120 on the first two cylinders and then got around 70 on the 3rd cylinder. Everything is stock on this ski, and it has been sitting for the past 2 years. (I just purchased it from someone else). I put some 2 stroke oil down into the 3rd cylinder and the compression shot up to 90. Any suggestions here? Can I keep running it the way it is, or will this tear up the motor? Should I rebuild all three cylinders with a top end kit, or just that cylinder. I just don't understand why it is running so well in the water with the 3rd cylinder having that low of compression.

    Compression test was done with a cold motor and with an OK gauge, but not the greatest. Did not do WOT.

    Thanks for any assitance you guys can provide
    First get a quality compression tester and check cylinders again.If rebuilding top end do all three cylinders.
    Last edited by Glenn Briglio; 07-04-2012 at 08:44 PM. Reason: spelling

  4. #4
    Yamaha artisan Cutlass's Avatar
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    57 mph on GPS? or Dream-o-meter? I'm guessing dream-o-meter.
    You're gonna want to fix it before it gets worse and hurts the crankshaft. Its possible that cylinder isn't getting enough oil or the crank seal is starting to leak which will cause that cylinder to run lean and get hot/hurt the rings and cylinder.
    IMHO, you should leak test the engine to see if the crank seal is leaking. If it is, then pull the engine out, replace the crank seals and redo the top end. Its best to do all three cylinders at the same time.

  5. #5
    Thanks for all the good information. Quick questions...how can I check the crank seal? What do I need for that? I think the cause is from the 3rd cylinder not getting oil, but not sure. There is oil down in the hull of the ski, so it is leaking somewhere. I am basically trying to trouble shoot this one, so need some steps. Also, anyone know where to get a good priced top end kit that is decent and will last?

    Thank you so much for all the replies, it really helps. I am parking the ski and going to tear it down before I use it anymore .

  6. #6
    I also plan on getting rid of the oil injection and purchasing the block off kit. Going to mix myself!!!! Then I know we have oil!!!

  7. #7
    Yamaha artisan Cutlass's Avatar
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    You can do a quick visual inspection on the rear crankseal by removing the cover over the coupler. If the seal is looks obviously bad, then you'll know. Otherwise you'll need to pressure/leak test the engine. If it doesn't hold air pressure, then you need to start spraying down suspect areas with soapy water and look for bubbles. Here's some links on pressure testing.

    Pressure Testing Two Stroke Engines

    GP1200 65U pressure test
    pressure Testing a 65U SUV
    Pressure Testing

    since you have oil at the bottom of the hull, start checking the oil lines to see if one popped off.

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