NS40C. No power

[Verne243]

I'll try to be short but concise...not easy given the situation.

Last year I bought a Nissan NS40C tiller steer motor (used 2 cylinder) to replace the 2 cylinder 25 hp Mercury I had on my Grizzly 1648.

The 25 did great with one but took a little to get on plane with two. It would do about 25 mph +/- depending on number of passengers and gear. I wasn't really looking for more speed, just more power to get on plane easier. Enter the 40 horse motor.

Since I've owned it, it has never run more than 23 mph and will barely get on plane with 2 people. Once you're on plane, it's as if you're only running at half throttle with low RPMs.

Since I bought it, I've done the following to try fixing it:
Replaced gas tank and fuel lines.
Had a tune up done and motor gone through.
Had a new carb installed.
Dropped the prop from a 3 blade 11.5X12 to a 4 blade 11.5X8.

None of this made any noticeable difference, though it does get on plane a little better with the prop change.

I've gone through every website and boating forum I can find searching for the answer. Finally I came upon something concerning mounting height.

I have a 20" transom and thought that since this was a long shaft, it was 20" too. Yesterday I found out that it's not. It's actually 23" which puts the anti cavitation plate 3" below the surface when on plane.

Now I'm planning to install a jack plate to raise the motor 3". My question is whether this should fix the problem or not. Some say yes and others say it won't.

Any suggestions would be greatly appreciated. It's also been suggested that I could be dropping a cylinder or not getting enough gas due to the wrong fittings or hose.

I'm really at a loss here. This motor has excellent compression and checks out in every way according to the marine mechanic.

 

[pvanv]

You definitely want to raise the transom height. Trial and error will be needed. The water surface should pass between the antiventilation plate and the splash plate. Luis (Sea Rider) has a very good description with photos. Throw a tach on it to see what the WOT RPM registers.

 

[Sea Rider]

I've had recently some ignition timing issues with new factory delivered 30 & 40 Tohatsu OB's, same as Nissan models. Could be set to less timing when carb is at it's max butterfly opened position. If out of correct timing it's one of the causes to produce a slow plane, other issue is that sometimes those 30 & 40 ara factory delivered with the incorrect medium pitch prop, OB will lose about 1 K revs on medium light applications. Are you using correct plugs, that's NGK BR7HS-10 or BR7HS-10 gapped tight to 1.0 mm ?

Once you have a healthy OB, can install an induction Hardline Hour/Tach, go for a wot spin lightly loaded and check achieved max wot rpm, check if inside factory min-max wot rpm range, then can go for a prop maximization to pull revs up and achieve a better hole shot by playing with pitches. Don't have experience with 4 blade props, but you can play all you want with 3 blade props, there are many to choose from.

Check timing advance firstly, if out of adjustment, adjust properly, if don't know how to check or adjust, let us know shortly, then can troubleshoot extensively to get that 40C impetus fully out.

Happy Boating