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How do I upgrade my Yamaha 9.9 to 15 hp ???

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  • How do I upgrade my Yamaha 9.9 to 15 hp ???

    Dear all.

    Im in doubt whether I can upgrade my Yamaha 9.9 2 stroke engine to a 15 hp. How can I determine whether the reed valve / reed housing is a 9.9 or 15hp ? I have read that the carb is the same. The previous owner mentioened that it might have already been upgraded but he wasnt sure. One thing I have noticed is that the engine only increases the RPM up to half throttle or 2/3 throttle. After that point it runs smooth but is seems like there is potential for more.

    Do anyone know how to determine the difference between 9.9 & 15 inlet parts ? Please take a look at the attached photos. Seems like the max movement / opening of the reed valve is approximately 6-7mm.

    Last but not least which parts is needed if I can upgrade ? Part numbers are welcome...

    Hope someone can assist.

    Thanks for your kind assistance in advance.

    All the best.

    Michael Back
    Copenhagen - Denmark
    Attached Files

  • #2
    info on what load,RPM, and prop might give some indication.

    maybe they make test wheels for 9.9 and 15 hp motors to run to make sure motor can turn it properly.
    Or try to find someone to run it on a dyno to see what HP it puts out

    you can always buy the 15 reed /box to see it is the same as what you have on it right now.

    In reality if it will do what you want it to, then no need to worry about it
    Last edited by 99yam40; 05-20-2017, 11:10 AM.


    • #3
      Need the complete model identity to better answer your question.


      • #4
        Thanks - try to look at the photos - where the model specs are stated on a plate...


        • #5
          Your motor should have a data plate on it. That data plate will include the complete model identity and the PID (primary engine ID). Looks like what is seen below.

          Now I can guess based on you giving me limited information as seen in your photos but the information that I provide in turn might very well be limited.

          Is it that big of a burden to provide the complete model? Help us to help you.


          • #6
            the only difference is the 9.9 used a two reed box and the 15 used 3.


            • #7
              So, from what rodbolt posted and your pics, you have the 3 reed box already


              • #8
                Yes, and note the three reed valve assembly has a 63W prefix. 63W being for the 15 HP motor. 63V prefix is for a 9.9 HP assembly. Same as the prefix for the carburetor.

                $65 bucks for the 15 assembly means a cheap solution to get 50% more HP.
                Last edited by boscoe99; 05-20-2017, 02:42 PM.


                • #9
                  same test wheel different RPM.
                  9.9 is 5000-5200.
                  15 is 5200-5400.


                  • #10
                    Must have a larger prop to handle that extra HP,

                    maybe too large if the motor cannot push it up to proper RPM.
                    or the motor has other problems


                    • #11
                      51% more power (15 - 9.9/9.9) on 42% more gasoline (1.93- 1.35/1.35). For $65 bucks. What's there not to like about this modification?

                      A bigger propeller may be needed but more than likely the extra HP would bring the present propeller into the desired RPM band. Given that most motors are over propped.


                      • #12
                        well OP said he was not happy with the no response after 1/2 or 2/3 throttle.
                        something will have to change to make him happy or at least he should find out what RPM he is able to get to
                        Last edited by 99yam40; 05-20-2017, 04:17 PM.


                        • #13
                          Originally posted by 99yam40 View Post
                          well OP said he was not happy with the no response after 1/2 or 2/3 throttle.
                          something will have to change to make him happy
                          I have heard that about a lot of Yams. At some point additional throttle does little to nothing.

                          Could the reed valve assembly be acting as a choke point not allowing air through it to the extent that more air might flow through a carburetor? Don't know.


                          • #14
                            Lowering air flow volume whether by a smaller set of reeds or a smaller carb would obviously result in less power...

                            Like a 2 bbl carb vs a 4 bbl Rockchester spreadbore...

                            As the 15hp has a slightly higher RPM range, the additional 5 HP may get him there just with the mod

                            Last edited by TownsendsFJR1300; 05-20-2017, 04:39 PM.
                            1997 Angler 204 CC, 2006 F150 TXR


                            • #15
                              RPM range is the same.