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Rough running at low RPM

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  • Rough running at low RPM

    I have a 2000 90hp Yamaha that runs rough at low rpms and is very hard to start when hot, compression is 115, 105, 115 runs great at higher rmps. I have spark on all three plugs but no 2 is not making a difference when I pull the wire when running at high idle and the plug is wet when I pull it. Any thoughts and thanks for any input in advance.

  • #2
    too much fuel or too little fuel to air ratio will keep a cylinder from firing/ burning.

    check you fuel pump for leaks into the crankcase if #2 feeds the pump.
    Pulling the cowling and silencer so you can see into the carbs while problem shows up might give some liknd of clue.
    and you can spray some fuel/oil mix into the individual carbs to see if that helps.


    • #3
      A couple of things, the pump as mentioned above, plus that model has quite a unique ignition system, number 2 spark is created seperate to 1&3. Have the carbs ever been properly cleaned? I guess you have replaced the plugs?


      • #4
        Yes plugs have been replaced last week, thank you for your replies. I will dig into things soon.


        • #5
          Another thing to look at on that motor related to carbs is the primestart system. it draws a vacuum from the #2 cylinder. if the primestart diaphragm is torn it will dump excess raw fuel into the #2 crankcase. Happened to mine a couple of years ago. very poor idle but ran fine once I got it on plane.

          I would check that and the fuel pump as stated above. if the fuel pump diaphragm is torn it will do the same thing. but I can not recall off the top of my head if it draws a vacuum from #2 or #3 cylinder. given it's location on the motor, I would guess #2