20HP Nissan 4 Stroke - "studders" at WOT

Discussion in 'Outboard Maintenance' started by gnuraider, Apr 12, 2010.

  1. gnuraider

    gnuraider Well-Known Member

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    Hi,
    I started noticing an issue with my 20HP Nissan 4 stroke last time I was on the water. Running anything but WOT, even when on plane it runs smooth as ever. At WOT it will start to "studder" ever so slightly. The first time I noticed it, it kind of felt like I was running into small debris in the water so I pulled over, raised the motor, inspected everything and was back "off to the races", but kept feeling the studder at WOT. I backed off a bit after getting on plane and it ran smooth as ever.

    Prop is almost brand new with no nicks or bent blades. Haven't had any collisions. The only thing I can think of is maybe bad gas. I only run non-ethanol gas in it so I have never used a fuel stabilizer or additive. It is a Gheenoe with 20HP 4 stroke so it sips gas. I use a gallon or 2 and then fill it back up after a few trips, but have never really run it to E. Just for kicks I have now dumped all remaining gas into the truck and will fill up with a fresh tank next time out, but I am interested in hearing input on other issues that might cause what I'm talking about and suggestions for things I can check into.

    Thanks!
    Dave
     
  2. Brett

    Brett > PRO STAFF <

    What is your top rpm per tachometer?
    Air bubbles causing prop to burp?
    Rev limiter kicking in?
    Spark plug misfiring?

    http://www.nissanoutboard.com/Nissan-20-hp-outboard-NSF20C1.html
     

  3. gnuraider

    gnuraider Well-Known Member

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    Thanks Brett - that is "almost" my motor...except I have the power tilt version with remote throttle configuration.

    As far as I know there are no bubbles...there have been no changes that would effect that since last outing.

    I don't have a tach, so I have really no idea what my RPM's are, so can't rule out the rev limiter being an issue.

    Spark plug misfire...would that misfire all the time, or just under WOT conditions? I am running like a charm even just a hair below WOT.
     
  4. Brett

    Brett > PRO STAFF <

    I've had plugs that worked great at lower rpms
    but misfired at full throttle. Easy fix, new plugs.
     
  5. noeettica

    noeettica Well-Known Member

    After Plugs go Up 1 Pitch on Prop

    What Prop are you running Now  ?

    Part Number You Want is

    3BAB645240

    9.25 X 11.5

    10 Pitch not enough

    Motor is Running better ... Boat is slipprier less wind ...gotta be somthing ...

    Dave
     
  6. gnuraider

    gnuraider Well-Known Member

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    Thanks - I will try the plugs and go from there. Been using a stock prop since I bought the boat and never had any issues until now.

    Just out of curiosity, how often do you all change your plugs?

    Thanks,
    Dave
     
  7. Brett

    Brett > PRO STAFF <


    Whenever the engine starts to misfire... ;)
     
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