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  1. #1

    1999 GP1200 took out a piston....again

    Been working on this '99 GP1200 for a customer for a few years now. Fixing it up, cleaning it up some, etc etc. They do take care of it and for the age its in real nice condition, IMO.

    Anyway, early last season it was running bad and they brought it to me. Did a compression test, #2 was real low. Turns out the oil line popped off the pump (wire tire broke holding it on like they all do), so that cylinder got no oil = meltdown. Took it apart, put a new piston, pin, and rings on all cylinders and got them to remove the oil pump and run pre-mix. Ran great all of last year.

    This year it fires right up after winter storage, and they run 1 tank of fuel through it, still running good. Following weekend its randomly cutting off until its really hard to start and wont plane.

    Bring it in, low compression on piston #1 this time. Pull it apart and piston is pretty messed up, theres even a small chunk missing out of the very top above the top ring, about 1/2" long.

    I blew the water lines out with air, seem to be free of shit. Took the carb apart and dont see anything that would cause it to lean out like that. I hate when shit like this happens and you cant find a for sure reason for it to know that its going to be fixed.

    Anyway, sorry its so long but Im just looking for other things to look for besides all new stuff, and re-ring the other 2 and rebuilding the carbs.

    I did not rebuild the carbs the first time b/c I took them apart and were very clean so I didnt think it was necessary.

    Thanks for any help and reading my novel.


  2. #2
    Yamaha artisan Cutlass's Avatar
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    air leak? pressure test the engine. Any pics?

  3. #3
    Myself's Avatar
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    I've got a feeling you'll find creased fuel pump check valves and a loss of fuel to that cylinder.

  4. #4
    I took some pics of the piston, I dont have pics of the case, but as you can imagine it needs to be honed bad.





    Quote Originally Posted by Myself View Post
    I've got a feeling you'll find creased fuel pump check valves and a loss of fuel to that cylinder.
    The only thing ive found "wrong" are one of the circle check valves in the fuel pump side has a couple real small messed up spots in it. I would take pics of that but its at my dads shop b/c I could get the damn screws out to get to the jets and make sure nothing was gummed up/clogged down there.

    After all the digging and only finding those small spots in the check valve, seems like it doesnt take that much to mess one of these things up?

    During the last rebuild I put a new fuel filter on as well.

    Thanks for the help guys

  5. #5
    Yamaha artisan Cutlass's Avatar
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    Quote Originally Posted by TooLateVTEC View Post
    After all the digging and only finding those small spots in the check valve, seems like it doesnt take that much to mess one of these things up?
    Do the "drilled returns" mod. It help keep those check valves alive and balances out the fuel pressure across all 3 carbs. Check it out here:
    Drilled Returns

  6. #6
    Do the "drilled returns" mod. It help keep those check valves alive and balances out the fuel pressure across all 3 carbs. Check it out here:
    Drilled Returns
    Gotcha, thanks for the link bud I really appreciate it.

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