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Loosing power/speed

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  • Loosing power/speed

    Hello one and all!

    Got an issue with my engine, hoping someone can help!

    Details as follows:
    1990 yamaha 4 stroke f9.9 (6g8) electric start outboard.

    Parts replaced already:
    CDI unit.
    Coil pack and leads.
    Spark plugs.
    Fuel filter.
    Fuel line from tank to engine.
    Oil sensor.
    Thermostat.
    I've also taken off the carb before and cleaned it, float seems okay and adjusted fine.

    I've taken off the prop and inspected, and regressed it. (I had originally seen something to do with prop spun but I don't have this setup judging by the 50+ videos on YouTube - I'm guessing it's an older style prop)

    Just a few things I've noticed which seems to be common - for example;
    Going along at "half speed" - it will gradually get slower, and slower - then slowing myself to a stop - putting it into reverse for 5 seconds, then back into forward brings the engine back to "good" speed again - then 20/30 minutes later do the same again.
    Water pees out fine and never an issue.
    Oil light never reports an issue.

    I would say the boat engine is running at just less than half power/speed as a basic statement.

    Idle is great, probably the only part I'm happy about.
    Other than that - I'm at a loss as to what the issue is.

    I've contacted 3 outboard mechanics locally (+/- 20 miles) and all 3 have not got back in contact after a few attempts - Clearly don't want the work haha.
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