1955 25HP Evinrude: How can I block off these ports?

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  • mr-asa

    US Member - 2 Years
    Replies: 794
    Topics: 138
    #243755

    #43 and #45 on this diagram. #43 is easy, that’s pressure line for the gas can which I’m not using. I’m using an external pump so I don’t need the pressure from #43, but I also don’t need it spitting air-fuel mix everywhere. Can I just connect the two?

    • This topic was modified 1 month, 2 weeks ago by mr-asa.

    crosbyman

    Canada Member - 2 Years
    Replies: 2229
    Topics: 233
    #243770

    just install short piece of fuel hose on the air pressure nipple and insert (screw) a short bolt in it to seal it and install tye wraps to hold everything in place

    Joining AOMCI has priviledges 🙂

    • This reply was modified 1 month, 2 weeks ago by crosbyman.
    • This reply was modified 1 month, 2 weeks ago by crosbyman.

    fleetwin

    US Member - 2 Years
    Replies: 4030
    Topics: 41
    #243850

    I’m confused, why are there two nipples on the intake manifold? Is one of them for the cut out switch??


    crosbyman

    Canada Member - 2 Years
    Replies: 2229
    Topics: 233
    #243852

    possibly… a vacum is needed … I modified my comment o the air pressure nipple

    Joining AOMCI has priviledges 🙂


    David Bartlett

    US Member - 2 Years
    Replies: 517
    Topics: 22
    #243859

    Don is correct. One is for the vacuum switch.

    David Bartlett
    Pine Tree Boating Club Chapter

    "I don't fully understand everything I know!"


    mr-asa

    US Member - 2 Years
    Replies: 794
    Topics: 138
    #243860

    What vacuum switch?


    David Bartlett

    US Member - 2 Years
    Replies: 517
    Topics: 22
    #243862

    Evan,

    On the 1954 motors there is a vacuum switch that shorts one set of points if the motor “runs away” in neutral. I think the 1955 motors had one also.

    David Bartlett
    Pine Tree Boating Club Chapter

    "I don't fully understand everything I know!"


    Mumbles

    Canada Member - 2 Years
    Replies: 5122
    Topics: 274
    #243882

    What vacuum switch?

    This one. The motor will run just fine without it but it is there to prevent a runaway situation.

    DSC00046


    mr-asa

    US Member - 2 Years
    Replies: 794
    Topics: 138
    #243888

    Ahh. Is a runaway condition in these something you have to worry about regularly?


    outbdnut2

    US Member - 2 Years
    Replies: 1603
    Topics: 83
    #243898

    I’ve seen this switch kick in if a motor gets rev’d too high in Neutral.
    Dave


    mr-asa

    US Member - 2 Years
    Replies: 794
    Topics: 138
    #243903

    Interesting, I would think that the timing plate would limit that.


    Mumbles

    Canada Member - 2 Years
    Replies: 5122
    Topics: 274
    #243928

    I think describing the switch as a rev limiter is more accurate than describing it as a runaway preventer.

    When a certain vacuum is reached, at high RPM, the switch closes grounding out the set of points it’s hooked up to. This kills the spark on that cylinder and the motor slows down. In an outboard motor runaway situation, the motor revs up on its own uncontrollably because of preignition and no spark is needed to keep it running. The preignition is usually caused by an air leak which leans out the mixture and raises the combustion chamber temperatures. As a result, the spark plug or a piece of carbon will become red hot which will ignite the fuel charge before its time. Turning the key off or pulling the spark plug wires off will not stop the motor as by now the ignition system is redundant and just there for the ride. Flooding the motor by choking it to death will bring it to a stop.

    A runaway diesel motor is a whole different ball game. When they runaway, turning off the fuel supply has no effect as they are usually running on their own crankcase oil. A blown oil seal in a turbo will cause this and the motor will keep screaming away until it either breaks up or runs out of lube oil. Either way, it’s not good for the motor. Yeah, I know, strangling its air supply will shut it down to but diesels usually have a robust air filter system and by the time it’s removed to gain access to the intake, the damage will have been done.


    Buccaneer

    US Member - 1 Year
    Replies: 4998
    Topics: 820
    #243932

    I think describing the switch as a rev limiter is more accurate than describing it as a runaway preventer.

    When a certain vacuum is reached, at high RPM, the switch closes grounding out the set of points it’s hooked up to. This kills the spark on that cylinder and the motor slows down. In an outboard motor runaway situation, the motor revs up on its own uncontrollably because of preignition and no spark is needed to keep it running. The preignition is usually caused by an air leak which leans out the mixture and raises the combustion chamber temperatures. As a result, the spark plug or a piece of carbon will become red hot which will ignite the fuel charge before its time. Turning the key off or pulling the spark plug wires off will not stop the motor as by now the ignition system is redundant and just there for the ride. Flooding the motor by choking it to death will bring it to a stop.

    A runaway diesel motor is a whole different ball game. When they runaway, turning off the fuel supply has no effect as they are usually running on their own crankcase oil. A blown oil seal in a turbo will cause this and the motor will keep screaming away until it either breaks up or runs out of lube oil. Either way, it’s not good for the motor. Yeah, I know, strangling its air supply will shut it down to but diesels usually have a robust air filter system and by the time it’s removed to gain access to the intake, the damage will have been done.

    Reminds me of the time I was working on a diesel JD tractor in Vo-Tech.
    Not sure what I was even suppose to do to it, or what was wrong with it,
    but I was standing next to it and test started it. It started “WIDE” open.
    By the time the instructor ran over, I almost had it shut down, by
    holding a motors manual over the air intake……. I think the air cleaner was off!

    Prepare to be boarded!


    frankr

    US Member - 1 Year
    Replies: 5861
    Topics: 49
    #243975

    I say mumbles is correct—as far as it goes. The words for the situation probably are not correct. I have spent lots of time thinking and pondering over the past 60 years about exactly what is happening in an outboard motor in so-called “runaway” condition.

    First, in an outboard the term generally means what happens when a motor is revved up in neutral or no-load situation, and cutting the throttle back does not slow it down. As suggested, it is not true thermal “runaway” It happens often in OMC Big-Twin and RD series motors and the sound is unmistakable when it is happening. The vacuum cut-out switch senses that condition and kills the spark on one cylinder till it slows down.

    So what is causing it to run away? In my theory and opinion, it is flywheel momentum and manifold vacuum. Those motors have large and heavy flywheels for smooth running. Get the thing spinning fast and close the throttle, and abnormally high vacuum is created in the intake manifold.

    Now consider the carburetor. It has the main nozzle and a large venturi. Great for high speed and lots of air flow. But slow it down and there is not enough air flow through the venturi to make it work. So a second fuel and air flow route is provided for the idle and intermediate speeds. You could compare it to a second, smaller carb. Briefly, it consists of a hole or holes in the throttle butterfly with holes in the carb body critically placed in relation to the butterfly in closed or cracked-open position. At idle, fuel and air flow through these holes are what is feeding the cylinders. The main venturi and nozzle really aren’t doing anything because of the slow air flow through that area.

    So…now you create an abnormally high vacuum in the manifold. That high vacuum sucks more air in through the carburetor. Enough more air to also draw in more fuel, possibly even some from the main nozzle. Bottom line is this extra air and fuel is enough to provide the “kick” that is keeping the flywheel spinning rapidly. Killing the spark prevents the extra fuel/air from igniting, and it slows down

    BTW, I also have had a couple of hair raising experiences. One was a 40hp Scott-Atwater (no vacuum cut-out switch). I had to pick up a tool and bust off a spark plug to slow it down. Worse, was a Diesel that I accidently got into true runaway. I was so panicked that I don’t truly remember how I slowed that thing down. Suffice to say to me that I shouldn’t be working on Diesels.

    That’s my story and I’m sticking with it. At least till somebody corrects me.

    • This reply was modified 1 month, 2 weeks ago by frankr.

    joecb

    US Member - 2 Years
    Replies: 485
    Topics: 62
    #244000

    Thanks Frank, that all makes sense

    Joe B

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