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8hp smothering problem?

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  • 8hp smothering problem?

    My 8 hp, 4 stroke Yamaha has a problem where the engine dies if the cover is attached. The outboard mounted to a sailboat that live in the saltwater. It runs fine without the cover but as I slowly bring the cover down on the engine, it's like snuffing a candle. I recently discovered it also runs fine out of the water with the hose attached and cover on. This motor has a hose attachment point and I tried running it with the hose attached while on the boat in the water. Same problem. Tilting the motor out of the water with the hose attached, it ran a bit rough but did not run down to quit. It's acting as if, when the prop is in the water, the exhaust is not getting out so it's venting into the cover so that, when the cover is attached, the exhaust smothers the engine so it can't breathe. However, apart from a small bit when I first start it for the day, I don't see and smoke around the head of the motor. It will run fine in the water with the cover off and it will run at near top RPMs without dying with the cover on. There are no visible leaks of water or fuel. It's a clean looking engine (2001) and a recent checkup showed it to be otherwise in good shape with good compression. Any suggestions would be greatly appreciated.

  • #2
    An exhaust system leaking to the inside of cowling would cause the motor to run poorly and/or shut down....


    • #3
      hi there happened to an old 7.5hp mercury i had, it turned out to be exhaust tube connection, between lower unit and and exhaust tube had burnt out, causing an exhaust leak


      • #4
        exhaust leak or not

        I think it might be something else. Today, I tried holding the cover over the motor. When I lowered it to within 3" above where it seats, the motor started dying. It seems to me it's an intake problem and one guy suggested rebuilding the carburetor. It just seems to "smother" much faster than the time it would take for the exhaust to be concentrated enough to create the problem. However, it seems, even slightly restricting the air intake by lowering the cover, is enough to make it wind down to stopping.


        • #5
          O.K.,just an idea...maybe something stuck in upper cowling air intake?...a bird nest?...hornet nest?....check it out...