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97 Yamaha 9.9 MLHV Backfire and cough

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  • 97 Yamaha 9.9 MLHV Backfire and cough

    I got this 9.9 in trade and the guy said it had cab probs it would start up but without a lot of throttle would die. I rebuilt the carb and rebuilt the fuel pump and now it will start right up idle and runs full throttle on the water really good. The only thing is that it pops and backfires alot at idle do I just need to adjust the low idle screw or could it be something more serious?


  • #2
    Re: 97 Yamaha 9.9 MLHV Backfire and cough

    is it a 2 stroke?

    it could the the idle screw but there are a few other things it could be also

    but a quick question have you flushed the tank since you got it from this guy? there is no telling how old the gas is or if it has condensation in the tank from sitting

    also the oil/gas mix might be off so flushing the tank will solve that to

    i would throw new plugs in it also...

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    • #3
      Re: 97 Yamaha 9.9 MLHV Backfire and cough

      If it is backfiring I would make sure the plug wires are on the correct cylinder, if it is a lean condition the idle mixture screw is just going to adjust the idle mixture, meaning probable carb overhaul.
      DO YOU HAVE: GOOD COMPRESSION, PROPERLY TIMED SPARK, PROPER FUEL AIR MIXTURE?

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      • #4
        Re: 97 Yamaha 9.9 MLHV Backfire and cough

        I changed the gas first thing i was gonna add some seafoam to the mix, I'm not sure but i have had a hell of a time finging a carb to manifold gasket for this motor, I have been to boat shops the internet even yamaha itself doesnt know if one even exists for it. The shop told me to put a thin layer of silicone or liquid gasket around it to seal it up but i think it may be taking on a little air where the carb meets the manifold i know that a lean mix will cause the fast sputtering and coughing maybe i should make my own gasket for it. oh and i rebuilt the carb exept for the needle and seat. If you see this and you own one of these motors maybe you can give me my answer as to wether or not there is a carb to mainfold gasket for this motor and yes it is a 2 stroke...

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        • #5
          Re: 97 Yamaha 9.9 MLHV Backfire and cough

          Originally posted by reggiebad View Post
          The shop told me to put a thin layer of silicone or liquid gasket around it to seal it up but i think it may be taking on a little air where the carb meets the manifold i know that a lean mix will cause the fast sputtering and coughing maybe i should make my own gasket for it.
          No silicone either make one or buy the carb repair kit, it comes with the carb gasket 63V-W0093-00-00
          DO YOU HAVE: GOOD COMPRESSION, PROPERLY TIMED SPARK, PROPER FUEL AIR MIXTURE?

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          • #6
            Re: 97 Yamaha 9.9 MLHV Backfire and cough

            are you sure that kit comes with the gasket because the diagram on ********** for that repair kit doesnt show a carb to manifold gasket

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            • #7
              Re: 97 Yamaha 9.9 MLHV Backfire and cough

              I looked it up on a generic site but in my experience the yamaha carb kit came with all necessary gaskets. To be safe compare price to a sierra cross reference part number.
              DO YOU HAVE: GOOD COMPRESSION, PROPERLY TIMED SPARK, PROPER FUEL AIR MIXTURE?

              Comment



              • #8
                Re: 97 Yamaha 9.9 MLHV Backfire and cough

                Checked it out but still couldnt find the gasket but i ordered the carb kit anyway just to see what it comes with, worse comes to worse ill get some gasket material and make one myself and ill have a rebuild kit on my shelf for the future. ill post if it comes with gasket if anyone is interested...

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                • #9
                  Re: 97 Yamaha 9.9 MLHV Backfire and cough

                  I got the carb repair kit today and it does not include a carb to manifold gasket and i cant find one that has a carb gasket, if anyone has any experience with this motor can you tell me if it actually has a gasket or what. Im just going to get some gasket material and make one thanks for the help and advice everyone.

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                  • #10
                    Re: 97 Yamaha 9.9 MLHV Backfire and cough

                    I just purchased a rebuild kit from my local boat shop. It came with everything (including the manifold to carb gasket) except the accelerator pump diaphragm. Apparently they sell those parts separately. Mine looked to be in good shape so I reused it. I don't know why your kits don't come with the gasket. The part # to my kit is 6G-W0093-03-00. Came in a OEM Yamaha box.

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