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50 HP two stroke lost RPM

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  • #1

    50 HP two stroke lost RPM

    Hello, my 50 HP two stroke is acting up again.

    A bit of history, Last year, boat ran great at 35MPH with outboard turning at 5500 RPM. About 6 weeks into last season, RPM maxed out at 4500 and top speed around 25MPH. Had the carbs cleaned and was back up to 35MPH and 5500RPM.
    At the end of the season problem started again, dropped to 5100RPM max and 28 MPH. I figured since the carbs had been cleaned it must be something else, replaced all fuel lines inside cowling and new fuel pump ( OEM).

    Put the boat in the water yesterday and RPM still topped out at 5100 and top speed 25 MPH. short period of time later after some low speed running I was only able to get to 3000RPM max and speed that wouldnt even register on the tach. I pulled the plugs and the top two had a lot of carbon (even though they were cleaned after the spring fogging oil burn off) and the bottom plug was wet and smelled of fuel.

    Prior to putting boat away for the winter I fogged the cylinders, drained the carbs, filled the fuel tank with stabilized fuel, changed the gear oil ( no water), replaced fuel filter element and replaced 10 Micron fuel water seperator.
    ;ast week ( prior to using boat) I pumped all of the old (storage fuel) out of the tank and filled with ethanol free fuel ( 91 octane as its the only non ethanol I can get).

    Is it the carbs again or could it be something else?

    -Spark tested and good
    -Compression 120 PSI on all three cylinders.
    -Timing was checked last year when I had the problem and was found to be good.
    Tags: None
  • #2
    If compression, spark, and timing are all good then I would lean toward the carbs

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    • #3
      Any idea what would be causing the carbs to continue to clog up?
      10 Micro Filter and fuel filter under the cowl were both clear and all new fuel lines under the cowl and fuel pump.. And fresh gas.

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      • #4
        Fuel-wet spark plug could point to a bad fuel pump diaphragm sucking raw fuel into that cylinder causing a very rich mixture....easy to check and replace. Good Luck!

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        • #5
          Fuel pump was replaced over the winter. Same problem with the old and new fuel pump.

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          • #6
            You need to test things now, do not think that the problem can not be something just because it was checked or replaced before. Timing light should show if spark is dropping and if timing is OK at the problem RPMs

            Puling and cleaning the carbs is not that hard to do, make sure you link and sync like manual says after reinstalling carbs

            Water or trash can hide in pump or lines and then hit the carbs and plug things up. I have found a piece of film from dried up fuel in the carbs I had cleaned several times, must have come from pump maybe.
            and also found a small piece of black fuel line that must have come off of a line from me pulling and putting the lines on and off so much trying to find problem that turned out to be someone had drilled the jets out.

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            • #7
              Thank you 99YAM40 as always... The boat is currently at the shop getting checked out. I expect to hear in a day or so what im looking at.
              I dont mind digging around and trying to do the carbs myself but id rather have a protake a look first.
              I was told by a previous mechanic that the Keihin carbs in my motor are a pain in the A$$ to clean.

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              • #8
                OK, I picked the boat up yesterday and took it out on the water and it ran great with RPM reaching the max and the speed of 35MPH that I expected.

                The mechanic that ficed it said he cleaned the carbs but they werent that bad ( although he said its sometimes hard to tell if a small passage in those carbs was clogged). HE checked compression, ignition and timing and all good.

                He noticed that there was a lot of soot at teh upper exhaust port which indicated to him that it was being over fuelled or oil. He checked out the electrical system and all came in within specs.

                He suspects a loose wire that he found that connects to the enrichment circuits may have been the overall problem with why the problem appeared after *****ing and gummed up the plugs.

                I'm going to hold off on *****ing for a little while to see if the problem returns at higher RPMs.

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                • #9
                  That would make sense if the enrichment system never turned off because of a bad connection. Hope you are good to go

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