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Thread: Nissan NS40D2

  1. #1
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    Default Nissan NS40D2

    Hi all,

    New to the site, love the info I have found so far.

    I have a Nissan(tohatsu) NS40D2 tiller on a 16 foot shallow v aluminum boat. It probably weighs 400-450lbs. the cavitation plate is about even to a 1/2 inch below the bottom of the hull. So far I have confirmed the stock timing settings and have adjusted the top carb throttle stop to allow the carbs to fully open (they were only opening about 2/3 stock).

    The motor runs well and has great bottom end power and mid range but seems a little flat on the top end.

    I have ran the stock 12.5 pitch large ear prop (little rake) and a 14 pitch mercury prop which was smaller diameter with more rake. The raked prop porpoised more and would not take as much trim.

    Top speed with 1000 lb load and empty was within 3 MPH (27 vs. 30 MPH) with both props.

    RPM's are unknown at this time (in process to install a tach) but the motor did not seem to be lugged.

    Empty top speed was somewhat limited by porpoising.

    To increase top speed do I need to?

    1) Take the reeds off the motor and set the reed stops to to 50Hp specs
    2) Adjust the timing to 50 specs (I adjusted the timing up 1 degree an it slowed 1 mph (18 to 19 degrees) I set the timing 1 degree down and is seemed to smooth out the upper midrange and gave me my 1 MPH back)
    3) Determine if I am bumping the rev limiter (when does it start coming in? is it a hard or soft rev limiter)
    4) Put larger carbs on the motor
    5) Look into a semi clever prop to give me some stern lift and settle the boat down and allow the trim to be more neutral (empty the boat still porpoises wthe the motor all the way down)

    Thanks in advance

    Mark

  2. #2
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    Keep trying--you're doing fine. You might consider adding some adjustable trim tab plates to hold down the bow.

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

  3. #3
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    think I would raise the motor up so you dont need any trim, get the motor running parallel to the water much easier if raised up a bit.
    Speed wise your not doing too bad for 40hp on 16ft boat.

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    thanks for the replys

    Are the reed stops which are set at about .250 on the 40hp vs .360 on the 50hp hurting top end power? I am probably getting more flow thru the carbs with them opening more.

    When does the rev limiter hit on this motor?

    It is my understanding that the M40D2 is a detuned M50D2.

    Thanks,

    Mark

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    If it's the same as the Tohatsu 50D2 which I believe it is, rev limiter comes in at 5850. The crank shaft is pressed, it's generally not recommended to remove the rev limiter without welding the crank first. Can't remember exactly what you need to do to go from 40 - 50hp but they are the same block. Check the part numbers on the carbs, it may be that the difference is in the jetting rather than the carbs themselves. Exhaust tuner may be different also. On a stock motor an 18" cleaver or semi cleaver will work well on a light hull.
    You get carbon reeds for them, from boyesen or Chris Carson. Can skim the head 1mm, will gives you ~30psi more compression.

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    Team Member Roflhat's Avatar
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    You can read my build thread with my old tohatsu 50, we get 63.6mph with that motor now on a thundercat with an 18p Mercury Chopper - http://www.boatracingfacts.com/forum...-My-Thundercat

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    Quote Originally Posted by Roflhat View Post
    You can read my build thread with my old tohatsu 50, we get 63.6mph with that motor now on a thundercat with an 18p Mercury Chopper - http://www.boatracingfacts.com/forum...-My-Thundercat
    Photobucket nixing the pics messes that idea up
    " Three may keep a secret if two of them are dead" Ben Franklin
    " ------- well Doctor what have we got a Republic or Monarchy? A Republic he replies if you can keep it"
    Benjamin Franklin, 1787 Constitutional Convention, as recorded by signer James McHenry's in his diary at the Library of Congress

    Location: SW Orlando, Fl

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    photobucket needs to be taken to court for what they done.
    they have messed up so much stuff, and lots of realy great threads with pics all the way through. we are talking thousands on this site alone.

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    Hi all

    The 40 carbs are smaller than the 50 carbs. The 40 carbs were only opening up 2/3 from the factory.

    I think I am going to go with Boyesen Reeds and remover the stops.

    Did the 50D1 tuner work better?
    Does anybody know how much bigger the 50 carbs are than the 40 carbs?
    5850 RPMs is pretty low... I suspect I might have been hitting the rev limiter. Got to get it out with the tach on it...

    Thanks again for everyone's responces

  10. #10
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    Is the 50D1 tuner enough of an improvement that it is worth taking the powerhead off to install?

    Thanks,

    Mark

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