Nissan 50HP Chokes at 6 to 7 mph

[The Tempest]

Our Nissan 50HP choke's when throttled above 6 or 7 knots. It just sputters when I try to push the throttle beyond 6 to 7 knots. I even had a couple of special service repairs completed, but no improvement. load is moderate. Former owner was easily able to get up to 19 knots. Ideas?

This is on the 26M. Plugs and air compressor replaced with out any improvement ($1,000). Impellor was also replaced with no improvement ($800).

[Jimmyt]

Welcome to the forum! Sorry your first post is about motor trouble.

If the motor runs fine at idle, then bogs down when you apply throttle, I would start by checking the following.

Fuel hose, bulb, fittings. Any small leaks in the fuel hose can be a real issue.
Tank collapsed/venting issue
Fuel pump
Carburetors

If you can clarify further about whether the engine runs fine below the bog-down speed; and add any other characteristics you can think of (did the motor sit for a long period of time since last use?); it might help in the diagnosing. It's difficult to do without actually being able to see the motor and do tests on it, but someone may have seen it before. The more specific you are about the description, the more likely someone might recognize the issue.

Have you, or the PO used ethanol gas in it and let it sit for an extended period?

Again, welcome. Hope you get it worked out before you spend many more boat bucks on it...

[Democritus]

Tank collapsed/venting issue

This seems super likely to me based off the symptoms. Check for vents that need to be unscrewed at the top of your fuel tanks.

[Russ]

Is the fuel old?
How is the fuel filter?
As mentioned above, fuel lines and tank vents?

Can you clarify "chokes"? Does it sound like it's starving for fuel?
Does it start to bog down or just won't go over that speed?

[Be Free]

Is this a 2 or 4 stroke?
Carburetors or injectors?
What is the (approximate) RPM when it begins to bog down?
Are there any alarms?

I know from your location you live at a relatively high altitude. If you are using it at altitude, has the engine been modified to run there? Was the previous owner also using it at high altitude?

[Starscream]

19 knots from a Nissan 50HP? Wow! Sounds like magic, I can barely do that with my 90HP. How well do you know this PO?

I would replace the gas lines including the primer pump between the tank and the motor as the first place to start.

[The Tempest]

Welcome to the forum! Sorry your first post is about motor trouble.

If the motor runs fine at idle, then bogs down when you apply throttle, I would start by checking the following.

Fuel hose, bulb, fittings. Any small leaks in the fuel hose can be a real issue.
Tank collapsed/venting issue
Fuel pump
Carburetors

If you can clarify further about whether the engine runs fine below the bog-down speed; and add any other characteristics you can think of (did the motor sit for a long period of time since last use?); it might help in the diagnosing. It's difficult to do without actually being able to see the motor and do tests on it, but someone may have seen it before. The more specific you are about the description, the more likely someone might recognize the issue.

Have you, or the PO used ethanol gas in it and let it sit for an extended period?

Again, welcome. Hope you get it worked out before you spend many more boat bucks on it...

The boat at about the same altitude, Salt Lake (not in the Salt Lake, but other mountain lakes) and Boise, ID where I purchased it. I've had it out on Puget Sound, sea level, with the same outcome.

As I push throttle up I reach an optimal speed of about 6 to 7 knots, pushing throttle up higher results in extremely diminished speed and higher still results in the engine sputtering and sometimes stopping. I have not heard alarms, beeps or seen warning lights. RPMs at dock revving can get up to a momentary 4,000, but cant get it up to 4,000+ RPMs on the open water. Most always use non-ethanol gas and stabilizer. I don't know the fuel used by the previous owner. I replaced the the fuel line and bulb without any improvement. We have from time to time forgotten to unscrew the gas tank vents, but this is not a normal practice. This engine is the outboard 2-stoke LTDI Nissan/Tohatsu 50HP.

Idle seems a little weak and has killed a few times while maneuvering in marinas.

[OverEasy]

Just a thought......
When was the last time the carbs were serviced/adjusted/sync’d?

Carbs can/do “wander” over time or get unintentionally adjusted....
and sometimes you think your getting one fuel but actually the other fuel was loaded into the station tanks leading to gummed up carbs....

[Be Free]

Correct me if I'm getting any of this wrong.

We are dealing with a 2 cycle direct injection engine.
That means no misadjusted carbs but it could be bad or dirty injectors

The problem has be around for a while.
You said it happened in Puget Sound and in the Utah mountains. We won't be able to tie this to anything that changed in the engine.

You have seen it at sea level and at altitude.
We can eliminate programming or tuning for high altitude use. (I was really hoping this was the problem )

Below around 4000rpm it works
Above around 4000rpm it stumbles and dies.
This could be lots of things. More below.

The plugs and air pump have been recently replaced.
Did the mechanic do this to fix the problem or was this just periodic maintenance?

The water pump impeller has been recently replaced.
Good to know but absent an overheating condition it probably can't affect engine speed.

You don't have anything unusual in your fuel tanks/lines like multiple tank selector valves, in-line fuel/water separator, in line fuel filters.
For troubleshooting purposes I'm assuming a portable tank with a flexible fuel line with a squeeze ball in the middle. If you have something more complicated then more things will need to be checked.

Back to the RPM issue. Lots of things can cause this. Most of them have been mentioned before but let's look at root causes. I can think of four things that can cause an engine to stumble or stall above a certain RPM.

Lack or excess of fuel.
Fuel lines blocked or sucking air at any connections. Squeeze bulb malfunctioning. Pickup in fuel tank blocked. Tank vent not working or closed. Fuel pump. Fuel filter/separator. Clogged or malfunctioning injector(s).

Lack or excess of air.
Dirty air filter. Air entering the fuel line at any point. Air pump (don't know this engine but I assume this is some type of mild turbocharger). Air flow sensor (present in most injected engines). Air entering the intake manifold anywhere it's not supposed to.

Weak or incorrectly timed spark.
Plugs bad, dirty, incorrect, or improperly gapped. Bad coil(s). Ignition timing off (sensor or ECM problem). Plug wires.

Engine protecting itself due to a real or imagined fault condition.
Any sensor that could put the engine into "limp home" mode. Oil pressure/volume/temperature, water temperature, 2 cycle oil level. I'm just guessing what your engine might be looking for.

That's what I can think of off the top of my head. I'm sure others will think of other things to check. I recommend checking on any work that you "farmed out". People make mistakes. You may not have the right plugs or they may be gapped wrong. Either one could cause the problem you describe all by themselves. Take the plugs out and look for soot or oil deposits. Look for anything that looks out of place or odd. A single vacuum line could also be the culprit.

If I were troubleshooting my own engine I'd do it in this order.
Verify that the right plug is in the engine.
Look for any problems with the plug wires.
Remove the plug and check gap and condition (soot, oil, etc.)
Check air filter. Follow the path the air will take looking for anything loose or cracked.
Operate the throttle and observe the movement of anything that controls the air flow.
Check the fuel filter/separator. Is it clean? Is there any water in it? Check your manual, there may be more than one.
Check all fuel lines paying particular attention to all fittings. The lines are under vacuum while the engine is running, not pressure so they are more likely to suck air than they are to leak while the engine is running.
Pump up the squeeze ball. Does it stay hard? Do you see any fuel leaking anywhere? You can usually follow the fuel line running from the back of the connector on the engine to the fuel pump. Don't forget to look there too. Do you smell any fuel?
Check the fuel tank vent.

If possible:
Access the fuel pickup in your tank. This will probably require partially disassembling your tank. If it is rubber, make sure it is not swollen, cracked, or deteriorating in any way.
Check any filters in the tank.

I'm sure I've missed some obvious things to check but it's getting late and my brain just shut down. Go slow. Be methodical. Don't take anything for granted. Good luck and let us know what you find.

[Tomfoolery]

I gotta ask the obvious question - are you sure the prop isn’t way too aggressive for this application? Maybe for slow speed cruising to save fuel?

[Jimmyt]

You say the boat is running at the same altitude as where you purchased it. Did it run correctly when you went on your test run at purchase? In other words, have you actually witnessed the motor running correctly at full throttle?

[Democritus]

Interestingly enough this same thing happened to me this weekend! I checked the vent and it didn't seem to be closed. I was going to go back to the dock to start looking over the motor but first thought to try my other fuel tank (and line). No problems at high revs this time. We motored around all day with no problems on that tank/line. I'm still suspicious of the tank vent because these new vents seem to keep pressure even when open. I might just cut it off and stick a bung in there when it's not in use. First I'll check the in-tank fuel strainer in case that is blocked up (both motor and tanks are 2020 so I doubt it) and also investigate the fuel line as well to see if it's sucking air. I've only had the boat in the water a handful of times since getting it in January but haven't had this issue on previous outings using both tanks.

[Jimmyt]

New tanks should allow vent in, but don't vent out - my understanding. Emissions reduction strategy. Check this article. If you have an old motor and a new tank, you may need to add a component to reduce flooding...

https://www.westmarine.com/WestAdvisor/ ... Components

[OverEasy]

Hmmmm!
The Fuel Pressure Device-to limit downstream pressure to the engine from a pressurizing tank..... well that’s something to add to our Fuel System Modification plan. Very glad you posted this Jimmy’s!

It will saved that embarrassed look that would have occurred on the afternoon return trip.
Cool morning start....Arrive at destination island beach .... warm sun and no engine run for trip home on an out going current... and the chief engineer and bottle washer with a perplexed look ....... .....

You just saved the day Jimmy’s!
Thank you!

[Jimmyt]

Probably more of an issue with carburetor motors, but thought I'd throw it out for consideration.