Nissan NS40D2

[fastcatt]

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

[Fastjeff57]

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

Jeff

[filthy phill]

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.

[fastcatt]

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

[Roflhat]

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.

[Roflhat]

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/forums/showthread.php?16744-My-Thundercat

[zul8tr]

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/forums/showthread.php?16744-My-Thundercat

Photobucket nixing the pics messes that idea up

[filthy phill]

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.

[fastcatt]

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

[fastcatt]

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

Thanks,

Mark

[filthy phill]

every little thing adds up, some tuners do restrict power big time.
look at mopeds today, most have a restricted exhaust, once removed they fly.

if the exhaust cannot get out the reverse flow causes huge power losses,
bigger carbs means more fuel, more fuel = more bang, but the big bang also needs to escape...
if you remove powerhead atleast you can see if any other restrictions are there like in the adaptor itself, plus clean it out of any salt or scale build up.
the powerhead is easy to remove and gasket is only cheap.
clean up the adaptor but dont go made in the exhaust area, smooth it out as much as possible to give nice flow.

instead of going to normal 50hp stuff, why not look into what the 50hp has done to it when in race trim ?
parts will probably cost the same or even be a touch cheaper to buy.

some of those 50hp Tohatsu / Nissan motors are bloody fast.

just a couple of thoughts

[fastcatt]

As far as putting larger than stock carbs, cutting heads and port timing changes. I am still trying to retain some tractability with the motor and fuel as it is on a fishing boat. I will probably leave the rev limiter on the motor to protect it for now. I build racing 2 cycle snowmobile engines so I am familiar with mods to 2 cycle motors. I am going to bore out the the 40 hp reed plates to fit the 50 hp carbs. I have noticed that the intake port on the reed plate is not centered over the reed blocks. Since I have to enlarge the intake port ~0.150" for the 50hp carbs I can angle the intake port to end up at the center of the reed block. The only problem is the intake port will be angled slightly in relation to the carb bore centerline. Would it be better to leave the original angle of the intake port and port the reed block to allow flow to the whole reed block or have a slight "kink" on the intake in relation to the carb and a centered port...

thanks,

Mark