Low RPM at WOT 2006 NSF8A3

[jmcst8fan]

Hey folks, hope you can help. I recently purchased a Gheenoe 15'4 (Highsider) that's powered by a Nissan 8 hp 4 stroke (NSF8a3). The stock prop had already been replaced with a Solas 3 blade 8.5x7 aluminum prop before I purchased it. The motor runs great, I haven't had any issues... fires up on first or second pull, idles within range (900 rpm) and seems to run a like a champ. My only issue is that the rpm's are topping out at around 3000 at WOT. Speed is right at 14 mph with just me (so less than 400 lbs total weight with boat, motor, me and gear). According to the specs, this motor should be running between 5 and 6000 at WOT. The motor is sitting on the transom w/ no jackplate, risers etc. Anybody have any ideas on why I'm not seeing higher rpms or any experience with these 4 strokes?

 

[TOHATSU GURU]

How do you know what the RPM's are? IE What kind of tach are you using?

 

[Sea Rider]

Yep, if by ear tach, it's no bueno LOL! Have you replaced current prop with a larger or less pitch prop from factory delivered one ? Factory delivered props are usually medium pitched, owner should go for a prop maximization installing a new prop for engine to rev middle to max wot rpm range as usually loaded. It's done right after when break en period is over or when buying second hand engines powering different applications.

Happy Boating

 

[jmcst8fan]

TohatsuGuru, apologies, I should've included this in the original post. I'm using a Tiny Tach model TT2A.

SeaRider, I have not tried a different prop other than the Solas listed above. That was going to be my next move, but it seems I've got a ways to go to get to even the low max rpm range on this motor. That is, of course, if my tach is reading true.

 

[Sea Rider]

If the TT tach is in excellent working order, red pick up wire was wrapped with min 3 wraps around spark plug cable and ground wire is well grounded, seems the tach was incorrectly installed as reads half of what needs to read. Set tach right to double actual rpm readings before venturing to play blindly with prop pitches..

Happy Boating

 

[jmcst8fan]

Wraps are good, ground is good. This was a brand new unit recently installed. The setting I'm using is 180 degrees. According to the instructions, this translates to 2 sparks per revolution. Is this the right setting?

 

[jmcst8fan]

Follow up: I contacted TinyTach and was told by a technician the setting should be 360 degrees for my motor.

I'll adjust and see where this puts the rpms. I'm assuming that would show higher rpm readings at WOT. This leads me to another question... that would put my idle rpm too high, is it Ok to adjust idle via the stop screw shown here labeled 1-22?




Thanks!

 

[Sea Rider]

When tach is correctly adjusted to 360° check which idle rpm is reading, readjust with idle screw turning it CCW to lower idle rpm if in need.. Sorry, can't tell which screw to adjust idle as don't work with 4 strokes engines for the moment.

Happy Boating

 

[pvanv]

No, that item 22 is the choke throttle differential stop. The idle stop is screw 12.

 

[jmcst8fan]

No, that item 22 is the choke throttle differential stop. The idle stop is screw 12.

Yes, ha ha, realized that when looking at motor yesterday evening. Thanks!

 

[jmcst8fan]

When tach is correctly adjusted to 360° check which idle rpm is reading, readjust with idle screw turning it CCW to lower idle rpm if in need.. Sorry, can't tell which screw to adjust idle as don't work with 4 strokes engines for the moment.

Happy Boating

Thank you for your help.

 

[Sea Rider]

You're welcome, yes the idle screw is the one with middle spring attached.. Good to learn that 4 strokes OB's works with 360ª firing, not 180º as 2 strokes OB's does. For the record, report which is the new wot rpm reading with tach set at 360º.


Happy Boating

 

[jmcst8fan]

Sea Rider, will do.

 

[pvanv]

You're welcome, yes the idle screw is the one with middle spring attached.. Good to learn that 4 strokes OB's works with 360ª firing, not 180º as 2 strokes OB's does. For the record, report which is the new wot rpm reading with tach set at 360º.


Happy Boating

Probably incorrect. The MFS8/9.8 has a "wasted spark on exhaust", and fires through a siamese coil, so the firing is the same as a 2-stroke. Further, if the idle speed were set too high, to say 1800 RPM, the motor would really slam when going into gear.

 

[Sea Rider]

If so, seems will need to go back to the drawing board again LOL. Will see what OP has to say as I assumed that the stated info was correct after having had a word with the induction TT manufacturer...

Happy Boating

 

[jmcst8fan]

Sorry it's been so long, but wanted to update. After adjusting the setting 360 on the TinyTach, my rpm's showed close to 2000 rpms at neutral idle. I adjusted the idle down to about 950 rpms and the engine maintained idle and; wouldn't you know it, was a lot quieter as well. It did seem to shake more, but would hold idle until I killed it with the switch. I have not had a chance to test WOT on the water due to work and water conditions on my days off. My local river is running super low due to lack of rain. I'll report back WOT rpm's when I have them.

pvanv- I did notice before adjusting that going into gear was a little rough, but having no experience with this motor, thought it was "normal". It now shifts so much smoother. Not 100% sure the 360 is right, but after my observations and talking to the technician with TinyTach, I'm rolling with it.

Thanks again, gang!

 

[jmcst8fan]

Final update: Had a chance to run it this past week. Minimal gear with my wife and I on board, we're probably close to 300 lbs combined. Tach showed 5850 rpm at WOT, max speed around 18 down river (current 1-2 mph). Thanks again!