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  1. #1

    1999 Yamaha GP1200 - Pistons look good and low compression on 3rd/rear cylinder

    Hey guys.....looking for some help here. I was getting around 115-120 compression on cylinders 1&2 (starting from the front of the ski) and on the 3rd cylinder I have a reading around 70. I squirted some oil down into the cylinder of the 3rd cylinder and then got a reading around 90, so I figured I probably need to go ahead and do a top end rebuild of some new pistons and rings. I was going to install an oil block off kit as well as a primer kit anyway, so I thought I would go ahead and do it all. I have started pulling apart the ski and after pulling the head off, here is what I saw (pics attached). Everything looked pretty good. The 3rd cylinder piston did have a few very very small knicks in the top of the head, could this cause the low compression? The other thing is that the 3rd cylinder seems dry as a bone, but the othere two have a black residue on them. Please see pics below. Any idea as to where I need to go from here? New pistons and rings for all three? Hours on the ski are right around 100. I am somewhat mechanically inclined, but by no means an expert, so please privide details if you have a suggestion/solution. I rebuilt a kawasaki 500 stand up before but there is a lot more here as well as another cylinder to work with, but I am determined to give this a go!

    Thanks so much for any/all help!!!!

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  2. #2

    More Pictures.....

    Here are a few more additional pictures.

    Thanks again!

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  3. #3
    Yamaha artisan Cutlass's Avatar
    Join Date
    Oct 2010
    Madison, WI, USA
    Yep, you probably need to do the top end. Finding number 3 cylinder "drier" then the other 2 is somewhat common. It can be caused by it running lean, heating up and burning off the oil. You need to check the rear crank seals for leaks. it common on these ski's as they age and that's what was wrong with mine.
    Actually I'd just suggest you pull the motor out, replace the crank seals (OEM only, aftermarket seals aren't worth it.), and do the top end. Rebuild the carbs using Genuine Mikuni kits only and do the drilled returns mod. If you thinking about ditching that big stock air box and putting on aftermarket flame arrestors, I have the perfect carb jetting and setup for you to use.

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