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Nissan 50 HP 2 Stroke NS50D2 will not rev over 2500 rpm under load.

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  • Nissan 50 HP 2 Stroke NS50D2 will not rev over 2500 rpm under load.

    I’m out of ideas on this one and need some help. I searched for everything related to this topic and cannot seem to figure out what the issue is.
    The outboard just does not want to rev over 2500 rpm under load. It will rev until about 2200 and then when I push the throttle down it seems like it wants to take off, but just hits a wall at around 2500 rpm.

    If I disconnect the shifter linkage and manually rev the engine it will rev all the way to redline, but not under load.
    I bought this motor from someone who said it has always been like this, so I have no background on it.

    Here is what I tested so far:

    • Right under 100 PSI on all 3 cylinders. #2 takes a few extra cranks to get to that mark, but they are all in same range. This seems a little low to me, but according to the manual the engine is supposed to have 103 on each cylinder.
    • Correct plugs, gapped properly.
    • Spark plugs are black and oily.
    • Sparklight tester shows spark on all 3 even at 2500 rpm under load.
    • Tested plugs grounded against motor and there is a spark jumping the gap. Possibly weak? Although I have nothing to compare it to.
    • Ignition coil resistance within specifications.
    • Exciter/Pulsar coil resistance within specifications.
    • Unplugged Black/Yellow wire for temp. sensor & oil sensor bypass on the CDI unit and there is no difference. There is no temp sensor on this motor, seems this was optional?
    • Could the CDI be faulty?
    • Fuel filter had some debris in the bottom of the canister, but I cleaned it and filter element looked clean. It was a bit deformed, but it still let fuel pass through the mesh.
    • Diaphragm in fuel pump was swollen and out of shape so I replaced it with a new one.
    • When I disconnect the main fuel line going into the engine housing and prime the bulb, fuel pours out generously, so I do not think there is an obstruction in the tank/main line.
    • Carb bowls fill with fuel to the top and fuel lines going into carbs stay pressurized when I prime the bulb.
    • At WOT I see fuel squirting into the carb throat from the main jet.
    • Tried squeezing the bulb while engine hits the cutoff, no change,
    • Tried running with open gas cap, no change.
    • Went through 2 fill ups.
    • Oil filter looks fine too and is properly adjusted.
    • Took the carbs apart and cleaned them. No dirt or obstructions anywhere.
    • No wear on any parts.
    • All gaskets good.
    • Tested float by flipping carbs upside down and blowing air into them and they work.
    • Carbs synched. Throttle linkage is good.
    • Timing was completely messed up on this engine. It was not even connected properly because of a coupler coming loose. Now the timing is set properly per specifications and the engine starts right up and idles nicely, but still no power under load.
    • Did not touch idle screw at all, but do not think this would be an issue.
    • Water pump works properly.
    • Prop is the correct one, but looks black and covered in soot where exhaust is supposed to come out of.
    Is exhaust supposed to come out through the prop or that little port underneath the cowl above the water line? I see exhaust coming out of that port hole the whole time, but when I block it with a rag I see the exhaust bubbling out from the prop. Is this an issue?

    One other thing that stands out. Someone took the carbs off along with the plate the carbs mount to, That metal plate attaches to the engine where the reeds are located. When they bolted it back together they used a gasket maker along with the original gasket. I see that in between carb 1 & 2 and 2 & 3 where the bolts hold this plate to the motor. There is no gasket maker or it almost looks “blown out”. There is a possibility that the metal plate and engine are flush in that area and the gasket maker was squeezed out completely, I’m not even sure if this would be a problem. Looks kind of odd to me, although, I don't think this allows air to enter into the chamber where the reeds are located. I will have to double check.

    One more thing I forgot to mention. There is no air box on this motor. I am not sure if there should be one, but the carbs are completely exposed under the cowl.

    I have not taken the lower unit off to look at it. Maybe there is a spun prop? Wouldn’t a spun prop cause an over revving engine and not accelerating boat? Instead I am having an under powered and slow boat, it feels like I have weeds wrapped around my prop.

    Also, the speedometer does not work. Not sure if this could somehow be related.

    I am completely out of ideas on what my issue could be. If you guys have some suggestions or other things I could test please let me know and thanks in advance. I am a newbie to outboards and just cannot figure this issue out.

  • #2
    Welcome to Iboats,

    All gaskets must be installed where they need to be working. Have you tested with a new fuel hose, connector's o'rings could be dried, cracked letting air in and OB missing specially if OB is very old...

    Personally would start with a prop slip test, probably shot or on it's way to become completely spun, then work from there. If OB runs good with no load, doesn't mean that will work same with load, load kills any OB performance if with tech issues..

    Happy Boating
    Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers


    • #3
      You MUST use the air silencer.
      3C8024110M AIR SILENCER $37.82
      3C8024180M RUBBER SEAL $8.67
      3C8024150M COVER, AIR SILENCER $26.73
      3C8024170M LOCK PLATE $4.25
      3C8024200M BOLT 5-16 PRECOATED $1.78
      3F3024230M TAPPING SCREW 5-30 $1.37
      You need 3 lock plates, 6 screws for them, 7 self-tapping screws.
      Buffalo NY USA


      • #4
        I will check the gaskets and do the prop slip test probably this weekend and report back.

        Getting the air silencer might take a bit longer. Could that be the problem though? People run open carbs on all kinds of engines. I think that whoever ran the engine with the air silencer off originally would have realized it runs horrible and put it back on.

        Any other things I should consider taking a look at?


        • #5
          Sign up today
          A new set of NGK plugs B8HS-10 or BR8HS-10 gaped to ( 0.9-1.0 mm, just in case...

          Happy Boating
          Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers