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99 SW250 OX66 Misfire

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  • 99 SW250 OX66 Misfire

    I can across a similar problem a few months ago and I thought I had it fixed. Yesterday it decided to act up again and I'm looking for some guidance. I have what I feel is a misfire at hig RPM. Anything over 4400 RPM it goes into a misfire mode just like a rev limiter in engaging. Engine/boat shakes violently until I bring the RPM back under 4400. Runs perfectly fine at lower RPMs. Anyone ever experience a symptom like this? or can provide some direction?

  • #2
    Replace fuel filters and check for water, try a new set of spark plugs.
    Regards
    Boats.net
    Yamaha Outboard Parts

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    • #3
      Thanks, The spark plugs will be my first check. Filters were all replaced. No sign of water in the sperator. I expect the plugs to be fine since I can get it up on plane and cruise at low spped. Once the throttle passes a certain point it starts with the misfire. I had issued in years past with LP pump failure, O2 sensor, water in fuel and this latest one has me stumped so far. When you guys have TPs sensors fail are the symptoms surging or misfiing? Also do you see coil pulser failures?

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      • #4
        Update on what I found. I checked the spark plugs and found cylinder #4 had a melted electrode. Detonation or lean condition? I was not able to test run due to the weather but I 'm assuming it should help. I checked compression on the entire bank and have equal 100psi across all of them. Next question. Injector issue? carbon?

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