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  1. #1

    Question Gp800 HELP!! Rear cylinder won't run... new top end, good spark,etc.

    1998 Yamaha GP800 LE

    Bought with bad cylinder. Replaced with good standard cylinder, still both nikasil, and new pistons, rings. Had crank checked, replaced crank front main bearing because it was a little noisy.

    Something stupid going on,only runs on one cylinder, the front cylinder.

    Both Cylinders have only 90 psi, not 120.

    Made sure power valve was adjusted per manual by lining up the hole in the bracket with the hole on the side of case.

    Already rebuilt carbs.

    Both spark plugs have good spark. Checked coil, etc.

    Can someone please tell me what the heck is going on, or rather suggest something else I can check?

    Rear cylinder appeared to have a coating of gas on the cylinder walls. I removed the head just to check gasket and look at cylinders, etc.
    Only think I haven't tried is manually feeding gas into the carb throat to see if other cylinder runs in case something got clogged in the carb when I rebuilt them.

    If the two cylinders had different compressions, it would be a no brainer, but with both being 90 I have no idea.

    Any help would be greatly appreciated.

  2. #2
    mudslanger's Avatar
    Join Date
    Jul 2010
    Huntsville Al
    Did you check the reeds. if you had cylinder damage you could have damaged the reeds. i would not run it until you resolve the problem. bad reeds will not build the proper crank pressure on the down stroke and also not let the fuel pump function properly. also did you make sure you had the base gaskets on properly.

  3. #3
    Put about table spoon of fuel into the cylinder and see if it hits

  4. #4
    I have considered reeds too. I don't remember seeing anything wrong with them, but that's a good suggestion.
    Pretty sure base gaskets are good now. I put one on wrong first time and when it went in water it immediately quit.. I had water jacket uncovered and it sucked water in crankcase. (stupid rookie move, I know)

    I purchased the ski with a bad cylinder, so I never ran it and did not witness the damage first hand when it happened. I will pull it back down and check out the reeds, but if anyone else has any input or has experienced anything like this, please feel free to comment.

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