Nissan 5 hp high-pitch whirring while full out before crash

NSF5A Ok, I need help.

Nissan NS5A, 5 hp 4 stroke, replaced the fuel pump, spark plug and cleaned the carburetor this spring. I only put 94 octane in it because that's the only stuff that has zero ethanol around here. It's attached to a 9 foot Achilles inflatable with wood transom and keel.


So the issue:


Only while the hull is planing, at 100% throttle, and top speed, with just me in the boat, after about 20-30 seconds there is a high pitched whirring noise that comes from the engine before it just chokes. (well chokes might not be the right word...she just stalls/dies/turns off)


What's frustrating about this problem is that it only happens when the boat is acting like a planing hull (not a displacement hull) and at high speeds.


The thing will run all day long at 100% throttle if there is 2 people in the boat. And I fished yesterday (trolling at low speeds) for almost 7 hours and I think this thing only drank a tablespoon of fuel. I love this thing...I just wanna go fast!!!!!


Sometimes after this happens, I can pull start it back up and she'll run ok in low-medium rpm.
But other times, I can barely keep it running by playing with the choke, and trying to find that throttle sweet spot that will just let me limp home.


I just don't know where to begin to troubleshoot this thing. Please help!!!

Is vent on tank open, Possible kink in fuel line,Check fuel filter if it has one. Running at wot all day is going to make that engine an anchor real fast.4 strokes can't handle this abuse. 2 strokes do much better at wot.

Is vent on tank open, Possible kink in fuel line,Check fuel filter if it has one. Running at wot all day is going to make that engine an anchor real fast.4 strokes can't handle this abuse. 2 strokes do much better at wot.What is age of motor,a little more history might help.

Hello, thanks for your reply.

I bought the motor from the original owner at the beginning of this year. It's a 1998 but has very low running hours. It was a sailboat kicker and came with a dirty carb and gummed up pump, which I promptly fixed. I did have some fuel/air leak issues early on after installing the new fuel pump.

Are you thinking a fuel or air shortage?

The NSF 4/5/6 (built by Tohatsu) will run WOT all day no problem. We have lots of them that do. The 5A/A2 has the older, more complex ignition system, which has an exciter, pulser, CD, and ignition coil. It's possible that those higher RPMs of planing might cause one of the electrical components to overheat, but I would not expect that to happen after only 20 or 30 seconds. I would be looking at the fuel system. Since you want to go fast, and since the only difference between the 4/5/6 is the carb, one way to go would be to order the carb for an NFS6A2, and gain 20% HP. That would rule out a flaky float valve, and also allow you an easier plane.

thanks for tips

You said it came off a sailboat, Is it a long shaft motor,possibly not compatible with your inflatable craft,Possible incorrect propeller,sailboat to inflatable?? When u said u cleaned carb was it removed,inspected,rebuilt thoroughly? There's the possibility some gunk still in fuel system. Was old gas from last year dumped??New fuel added to old?? Tank cleaned recently? Hope this helps!!

Hi guys, thanks for the input.

Paul...I'm going to buy a new carb. I'm tired of troubleshooting the fuel system. I've fine-toothed-combed every component from the tank to carb and I am confident it's not an air leak. I did have a few air leaks, which I resolved, and the engine seemed for perform for a little while at high speeds which led me to believe I fixed the problem...but nope...still reproducing the crash after high speed of 30 seconds to a minute.

Anyway...I had a tech buddy over who had a look the motor today AFTER I reproduced the problem (Ended up limping home with full choke, just babying the throttle to find that sweet spot where she wouldn't stall, sometimes she'll let this happen, other times I'm rowing home).

So he tinker's with Pilot Jet #42 (unscrews it slightly) while the motor is running (but wanting to stall)...and low and behold...she stops coughing and starts to smooth out. WHAT THE HECK?

I thought this screw was NOT "set-able" (part #3H6-03254-0) and my buddy agree'd. But he concluded that because playing with that jet screw settled the motor down in a good way...I probably have some problem with (perm varnish? scratches from cleaning? replace carb?)

Can you comment on this? I will be throwing in the towel soon...and if I get the 6A model carb and this problem continue's...I'm gonna cry.
What really gets me, is that if I just let the motor sit for 4 hours, it will resolve itself...Could this be an air bubble stuck somewhere?

Again thank you all for your input...and I apologize for my ignorance with this matter.

Cheers,
Jeff.

Jeff,

I suspect that the carb needs a more thorough cleaning. There could be a bit o junk loose in there that gets sucked into a passageway at high flows, and then settles back down.

Yes, the jets are fixed. However, trying to unscrew one changes a lot of geometry and flow characteristics. As I suggested back some time ago, I vote for a fuel issue, rather than electrical.

Precisely what method did you use to clean the carb?

How does this happen to a bowl in 4 months? Wow.

I've only put zero ethanol in her since I last cleaned the carb in April or so. I suspect it's biological growth at the fuel/water interface (crystally flakes on the paper towel). Maybe there was some leftover ethanol fuel in the primer bulb and lines? Maybe from fishing and throwing a net around the boat I've dripped water into the air-pin-hole on the tank? Maybe from the pacfic northwest humid/salty air? I really can't figure out where that much water would have come from to create that much crystal-gel in the bottom of the bowl.

I've used the boat and motor a bit more thank normal these days, like 4 or 5 times a week, opposed to 4 or 5 times a month. The outboard tank is 3 gallons...And she drinks about a tablespoon of fuel over 6 hours at trolling speeds...so how the heck am I supposed to keep the fuel fresh???

I've gone balls to the walls with this fuel system. Tank emptied, cleaned, rinsed with 99% isopropyl alcohol. Tank pickup removed, cleaned, sprayed with compressed air. All hoses replaced. Primer bulb replaced. Quick-connect replaced. New in-line paper filter installed (original was metal-mesh type, but salesman said there was a minimal pressure drop across this paper filter). New pump installed (bought 2 last April and I'm installing this one so I can have the peace of mind AND open the old one up and have a look).

As for a carb clean, I did a complete submerge of the disassembled carburetor in "Sea-Foam Auto Marine" for a little over 12 hours. I let the jets and screws sit in the solvent for 24 hours. Then I sprayed everything with compressed air and re-assembled.

So here's where I'm at...Thanks for your help everyone...Paul you are obviously a good tech. I will update soon when that fuel tank is completly dry and I can take her out for a spin.

That residue is clearly what we see in ethanol fuel when contaminated with water, when running "ethanol fix" types of fuel additives.
It typically does not happen if the carb is run out at the end of each and every day.
We never saw it before the introduction of E-10 fuels.

One thing I see here... is that we have non-ethanol available (I think ours is 91 octane)... and the fuel comes out of the end of a 15-foot long 1" diameter hose that is connected to a pump that allows you to select from 87 E-10, 89 E-10, or 91 non-E... Meaning that if the previous customer selected 87 or 89, the hose, pump, and mixing valve are all filled with that E-10 fuel. Maybe a gallon or two of it. Then one fills a 3-gallon tank, getting, say, 50% non-ethanol, and 50% ethanol, or "E-5" fuel.

I would suggest that... if buying non-ethanol gas fuel from a multi-grade pump, it would be best to select the non-ethanol grade, and then pump a couple of gallons into the car tank before filling the outboard tank.

As for keeping the fuel fresh, once it gets to be 30 days old, we dump it in the car and refuel the small outboard tank.

We have not had any significant cleaning results with sea foam. It works about as well in our shop as waving a magic wand. We use real carb dip, in 1-gallon cans, from the auto parts house. It is not nearly as aggressive as the stuff we got 30 years ago, (the stuff with oil floating on the top to reduce evaporation), but We usually go for about 4 hours (or more if cool temperatures), and then spray out with a whole can of generic acetone-type carb spray (to dislodge any organic varnishing that was softened in the dip).