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2009 Yamaha 8MSH new carburetor = runs slow

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  • 2009 Yamaha 8MSH new carburetor = runs slow

    Ahoy. We have a 2 stroke 8 HP 2009 8MSH. We inherited this engine when we bought a sailboat in 2020. It had been neglected for some time before we bought it and we have no information about it's prior history. We don't use the outboard very often; it often sits on the stern rail for several months between uses.

    For the first couple of years, it would run great and could easily get the dinghy planing with 2 people aboard, but only if we changed the fuel pump diaphragms before each use. Without new diaphragms it would bog down and stall when it was in gear and throttled above a slow crawl. To keep it running, we had to play with the choke to control speed instead of the throttle. Even if we only ran the engine once for 10 or 20 minutes, the next time we opened it up the fuel pump diaphragms would be all distended and wrinkly.

    We took it to a Yamaha mechanic who suggested changing the carburetor. So I bought an aftermarket carb on eBay and installed it. I also went through the maintenance procedure to set the length of the timing and throttle cables. Now it starts easy and runs well every time. But the throttle tops out at something like 50%. Turning the throttle up further sometimes causes a slight decrease in speed. I've checked the jets (even swapped them with the ones from the old carburetor) and replaced the fuel filter. Measuring the resistances of the ignition module gave readings that don't match the maintenance manual I have, but a spark tester on the ignition cables shows spark at both plugs.

    Any suggestions what to try next? It's definitely an improvement to have an engine that runs every time without a new fuel pump. But planing was fun and we miss that.
    Thanks in advance.



  • #2
    I'm not an expert on rebuilding carbs so I can't offer any good advice there - other than that would be a good place to start to make sure it's good. Are ALL of the passageways clear? If you're positive the carb is good (could be a bad one - especially from eBay), then look to fuel supply.

    BUT... I don't understand the reasoning behind replacing the carb to fix faulty fuel pump diaphragms? Besides, those should NOT be going bad in that short time. Maybe you're getting knock-off diaphragms that are of poor quality?
    2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
    1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)

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    • #3
      you say runs slow,
      do you have any Idea what the PRMs are when running well and then when running slow?
      have you checked compression, and timing of the spark to make sure they are in spec?
      is it possible the motor is going into RPM reduction mode do to over temp or low oil level?
      or if the motor thinks one of those is happening even if they are not really a overheat or low oil level due to a sensor or wiring problem.

      The CDI could also have a problem if you can see a timing is a problem when the RPM drops but everything else is OK .

      if you have good compression, good spark at the correct time, and advances properly staying in spec, then you would be left with a fuel to air ratio problem

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      • #4
        Is there an RPM reduction mode on a carb'd 8HP? I honestly don't know one way or the other.
        2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
        1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)

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        • #5
          I would think they all had the same type of motor protection.
          they all should reduce RPM to let you know there is a problem that you need to address but I have never played with an 8hp before.
          probably has a light to indicate

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          • #6
            Originally posted by 99yam40 View Post
            I would think they all had the same type of motor protection.
            they all should reduce RPM to let you know there is a problem that you need to address but I have never played with an 8hp before.
            probably has a light to indicate
            How that would work with a carb'd engine? I mean, there's no computer (like a fuel injected engine) to receive a sensor input and then throttle back the fuel delivery?
            2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
            1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)

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            • #7
              Originally posted by DennisG01 View Post

              How that would work with a carb'd engine? I mean, there's no computer (like a fuel injected engine) to receive a sensor input and then throttle back the fuel delivery?
              the CDI even on my old c40tlrx (1999) 3 carb motor controls the spark timing and other things

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              • #8
                Originally posted by 99yam40 View Post

                the CDI even on my old c40tlrx (1999) 3 carb motor controls the spark timing and other things
                Oh, right... the other element needed for combustion... spark

                IF the RPM was being reduced (I'm still skeptical ) then it would happen at exactly the same RPM each time. But this may be hard to judge via sound only.

                Question... if spark was being limited... wouldn't the engine sputter at that 50%--ish level that Hot is talking about? Just wondering outloud, here.
                2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
                1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)

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                • #9
                  But some sensor would still need to feed the CDI... and I didn't think a CDI was smart enough for this reason (doesn't have programming for this purpose)?
                  2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
                  1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)

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                  • #10
                    it is built into it,
                    over temp ,& low oil level switches hook into the CDI.

                    Mine even has motor warmed up switch to tell the CDI to go into normal running timing.

                    is this a premix motor ,if so then no oil level switch

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                    • #11
                      Oh, interesting. Thanks!
                      2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
                      1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)

                      Comment


                      • #12
                        The theory on replacing the carburetor was that it was likely gunked up from sitting around for months at a time with California's ethanol-enhanced gasoline absorbing moisture from the air. It's a 2 stroke engine, so the fuel tank is vented to the atmosphere. And impeded flow through the carburetor would both (1) put extra stress on the fuel pump, and (2) turn a weakened fuel pump into a bigger problem.

                        It runs smooth now, no misfires and hiccups; it just stops increasing revs above half throttle, and sometimes will slow a tiny bit when we max out the throttle.

                        Since it's a pre-mix 2 stroke, there's no oil level sensor. No temp sensors. No warning lights. The only things connected to the CDI are the ignition coil, charge coil, pulser coil, and kill switch. Those and the spark plugs are the only electrical components in the whole engine.

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                        • #13
                          something has to advance the timing.
                          hard to believe this new of a motor would still be moving the timing plate under the flywheel

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                          • #14
                            Might be hard to believe, but it's true. Timing is adjusted mechanically by the throttle mechanism moving the timing plate.
                            Last edited by hotatemaru; 01-17-2025, 12:18 PM.

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                            • #15
                              Originally posted by hotatemaru View Post
                              Might be hard to believe, but it's true. Timing is adjusted mechanically by the throttle mechanism moving the timing plate.
                              No not hard to believe at all. My best guess is the timing is not advancing enough. You said you had to adjust this when you changed the carb out? Did you put a timing light on it and make sure it was fully advancing?

                              Try and find some non ethanol fuel and get a OEM fuel pump diaphragm. Check butterfly in carb is opening all the way.

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