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Running rough in middle throttle / RPMs

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  • Running rough in middle throttle / RPMs

    Hello Everyone... Hoping my post comes through!...

    I have a 2016 DF90a Suzuki 4-Stroke outboard that seem runs great in Neutral Idol, a slow creeping no wake speed, and at full throttle she runs like a champ!... When I am in slower speeds going through no wake zones, About 35% of the throttle speed she seems to vibrate almost like a cavitation. The motor doesn't violently jerk like a misfire and it doesn't bog down like fuel starvation... It just vibrates the entire boat. If this makes sense, it seems to be an up/down motion...

    The Motor was bought brand new and was broken in by suzuki specs. She ran like a dream for the first season, did the scheduled 80hr / 100hr/ and 200hr maintenance. My uncle (owner)let it sit for almost 6 months before I was able to use it again. We did have water in tank issues after this time period. We had it cleaned professionally and she worked great for the season. The next season it sat for another 6 months and the VST pump (high pressure Pump) stopped working. I replaced the entire VST Unit, spark plugs, fuel filter twice, and water separator filter twice. I cleaned all fuel assembly hoses and checked the low pressure pump. The water in fuel situation seems to be okay and I carefully monitor it. She ran great this last season and I ended up keeping the boat so it doesn't sit.(first problem)!. Slowly the above issue started taking over and appears to be getting worse.

    I suspect it to be:
    low / weak fuel pump?
    Ball valve?
    spark plug / bad fire? Gaps?
    bad injector?
    Bad Prop?
    I can do everything for a Pretty Penny! Hopefully someone out here can help me pinpoint the issue!...

    Thanks for your time!.. JoeVan

  • #2
    I did replace the Propeller. It seems to decrease the vibration but it still does it around 20 -25 thousand Rpms. The Tachometer doesn't jump at all. Maybe this will help stir up some conversation?


    • #3
      Can you replicate the vibration in neutral? Trying to figure out if it’s the engine or the lower unit. Worsening over time has me thinking engine, maybe a bad coil?


      • #4
        Simplest way to diagnose this is going to be to hook it up to SDS diagnostic computer. It will save a lot of time and expense throwing parts at it.