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Yam GP800r initial thought are needs rebuild !

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  • Yam GP800r initial thought are needs rebuild !

    Bit of history...
    Not had it long..was running great first few times. 112kph on the speedo (58mph real speed gps)
    Next few times out started surging at top end. Would give the full beans sometimes and sometimes hold back and only do 90kph on speedo and dropped about 400 rpm.
    Next and time out...wasn't giving the full beans at all. Just 90kph max.
    Now, I suspect if I didn't know what it was like the first few times out, I would be none the wiser and just think that's what it's like and all is ok. But I knew it just wasn't right.
    So thought process was...fuel or spark problem.
    Replaced plugs, filter and fresh fuel after draining tank. Seemed to be ok but water was choppy and flat out speed tests weren't really possible.
    Last time out it was just the same. 90kph and noticbly more sluggish out the hole.
    So tonight (at 10pm) I did what I had been avoiding since buying it....a compression test.
    Now, I did this dead cold, because it was late and didn't want to annoy the neighbours.
    Rear cylinder was 120psi and forward 95psi.
    Fearing the worst here, but I will go to sleep hoping that it's just because the engine is cold and lacking a bit of oil in the bores.....but in reality I know it's probably buggered.
    It's likely I have a winter project on my hands and a top end rebuild.
    Easy enough we thinking ? I hear that I can send away the liners and get the pistons sized and sent back. Would they hone the liners too ? Don't have any experience with these wee engines.
    Any guides with pictures would be great !

  • #2
    The first thing I would do before even diving into rebuild mindset is find out why you have lost PSI on your front cylinder. Rear seems to be exactly where it needs to be - generally 110-115 psi is normal. You were right to do this cold as a warm engine will have piston expansion creating a tighter seal against the cylinder wall and giving a false reading. Some recommend holding throttle wide open while cranking and crank until your gauge stops increasing in PSI, I never have quite understood the purpose of holding throttle wide open as I have never experience a different reading doing so.

    Onto what the problem could be - oil line failure, incorrect oil ratio (or are you pre-mixing?), carbs need rebuild or are not tuned properly? Power valve failure? Those would be your three most common issues. I am not 100% certain of the GP800s but if they have the accelerator pump then the carbs are potentially your culprit as the accel pump from the factory has a lean issue in the mid throttle range due to not being reengaged from dropping enough throttle before squeezing another handful.

    Another factor is your cylinders should be Nikasil coated so that basically rules out a simple overbore and hone, you would either need to press out the sleeves and go with cast sleeve inserts or send them off to be replated and then of course another option would be a cylinder kit. You may could start with pulling your head and inspecting your pistons/cylinders internally, also throw up some pictures of your findings and motor.


    • #3
      Hi override

      I found the problem.
      It was the old broken off power valve ploy !
      Surprisingly didn't do too much damage to the cylinder...just to the piston.
      I stripped the top end last week and just waiting on new power valves, over sized pistons to suit the re honed cylinders.
      All going well I should have the parts back on Saturday.


      • #4
        Good deal! You might look into the carb issue I mentioned, if those GP800s are affected by the accel pump, there is a way to go about getting that back in order.

        Edit: Check out a product called Wave eater clips = CHEAP insurance to help prevent a future power valve failure.
        Last edited by override; October 5th, 2017, 09:45 PM.


        • #5
          Thanks. I don't think this would have stopped them breaking at the shaft.


          • #6
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            Quick update !
            After the rebuild and mostly now run in...the wee ski is running like a champ now.
            Looking into the wave water clips over winter !