Nissan/Tohatsu 5hp 4stroke fuel pump or carb issues?

Hello All,


I'm troubleshooting my recently purchased 8 year old 5hp 4stroke nissan outboard due to no extra power output after about 60% throttle while in gear. It used to bog down and stall out after 60% throttle and then I cleaned the carb and got brave and cleaned the fuel pump too. Now it won't stall anymore but it won't get any more power past 60% throttle. I noticed the diaphragms in the fuel pump seemed a little too stiff (like not as rubbery as they once should have been). And I read in this thread a guy saying that a marginal / bad fuel pump would limit top speeds and not idle speeds (the engine idles just fine).



On my last run out...I tried pumping the ball on the fuel line a few times while at full throttle. Well about 2 times out of 5 times trying that...the boat just TOOK OFF...faster speeds than I'd ever seen before...sea state wasn't the greatest so I had to turn her down each time she took off like that to avoid going airborne. But since then I haven't been able to reproduce those speeds/RPM.

So fuel pump or carburetor problem? I bought a pump for $40 and it should arrive in a week or two...Crossing fingers that will fix the problem because the cheapest carb I have found will cost me about $200 CDN with shipping.

I also read in that previous thread that the carburetor assembly for the nissan 6hp (3R4032001M) will fit in the 5hp and give 20% more power. I think I'd like to try this out. Anyone here done that or is familiar with that?


Thanks very much for your time and expertise.
Cheers,
Jeff.

I would definitely check over the whole fuel system with a fine-toothed comb. Clean the carb again in case there is some restriction in the high-speed circuit. Your float height in the carb may be a hair low, or the fuel pump may not be delivering a really strong delivery. Likewise, even a tiny air leak on the suction side of the fuel pump can cause air to be pumped in preference to fuel. Of course, any restriction in the tank strainer or the fuel filter will reduce fuel flow as well. Try a new spark plug, too. Yes, the carbs for the MFS4/5/6 interchange. The best one for your motor is:

If you are replacing the fuel pump, I would recommend that you make sure to get the pump that fits the 9.8, as the newer versions are improved:

Hi Paul...you are actually the same guy I quoted from a few years ago in that other thread! Thanks for your help.

I do have a few questions for you if you would be so kind...

-How do you find a tiny air-leak before the pump? Would this investigation need to be done at sea and in gear? or a garbage can of water in the backyard? (would be much easier for me)
-The carb looked good after I cleaned it...Jet passages with a very fine wire...seemed good... but I guess there's no such thing as over-cleaning?
-Also the float height seems completely level with the unit, and from what I've been told...level is what you want right?
-I do have a spare spark plug to try if the new pump doesn't fix things...but doesn't a spark plug either work or not work?

I really appreciate all your suggestions. I think I will install the pump first with another good cleaning of the carb and take her out on Friday. If she still doesn't perform I will start some of the other things you suggested (strainer/filter/and new spark plug)

Thanks again,
Jeff.

Jeff,

Air leaks can be tough to find. Air can leak through a small imperfection much faster than fuel. Sometimes an o-ring on the end of the primer hose can weep a little air, but the problem may go unnoticed for quite a while. If the leak is on the suction side of the primer bulb, it's common to have a "mushy" bulb... but at the motor connector of the primer, you might not find it easily. I usually get everything very clean and well lighted (outdoors on a sunny day is best), and then prime the bulb very hard... and inspect all hoses carefully.

I never advise using a wire to clean carbs. Even soft copper wire can make micro-scratches, which can make weird turbulence in the passages. The best system is to do a complete disassembly, a 4-hour soak in carb dip, and then a liberal blow-out with generic carb spray. The force of the spray clears any crud that was softened by the dip. Grab a magnifier, and verify the jet numbers as well, just in case the PO or mechanic swapped one of them. There could also be a "chunk" of crud trapped in a passage, moving around like a marble in a game.

Yes, you want the float level. Also make sure the needle is free in the inlet seat. Some oxidation can cause it to bind, and either not open or close easily. If there is white oxidation on the aluminum body of the needle (from water contamination), it probably should be replaced.

I have had some interesting spark plug problems over the years. Recently, my snow blower (air cooled Briggs motor, Champion plug), was running as though possibly too rich... then as though too lean. The plug looked OK, but used. I replaced the plug, and everything else straightened out. Turns out the resistor inside the ceramic of the plug was breaking down.

If you can't duplicate the problem in a test tank, you will need to test on the water.

If it were my motor, I would automatically switch to a 6 hp carb.

Very happy to be in touch with you. Thanks for the great advice...I will update this weekend as to my findings.

Cheers,
Jeff.

Hi Paul,

So I replaced the fuel pump and the spark plug on Friday, and today in a garbage can I am getting consistent, yet undesirable results. She idles for 3-5 minutes before stalling out. But not if I squeeze the priming bulb a few times. If I didn't, she would stall. If she was about to stall, I would pump the ball and she would even out and continue to idle. I reproduced this consistently.

Now what the heck? My first step was to clean the carburetor and replace the pump due to marginal/limited top speeds (didn't have this problem before). My next step was to thoroughly clean the carburetor, again. Hey why not throw a new spark plug in there too?

So did I mess up the carburetor? the float on the carburetor? It looks ok to me (I'm no outboard guy). I know they are delicate parts and I was pretty delicate when cleaning it. But after replacing the pump...and not having this problem before...I'm frustrated.

So it's clear the bowl in the carburetor is not staying full of fuel right? Is the carburetor not producing the venturi effect needed to keep the bowl full? is this a bowl needle problem? The needle and bowl "LOOKed" ok. They seemed to "operate" with the touch of my finger ok. The fuel system from the screen in the tank to the carburetor "LOOKS" good too. I even sealed a few connect points with tape hoping to eliminate any air leaks if present.

So does this problem describe what could be an air leak? or do you think a new carburetor fix me up? I'm only reluctant to replace the carburetor because it's a $200 expense that I just cannot afford at the moment. And I'm already bummed I bought a pump and replaced a spark plug that didn't fix things. If I buy the 6A carb and I still have problems I will be at wits end.

Any other suggestions all powerful outboard guru? It's such a beautiful day out there today in the Pacific N.W. I would love to be out ripping around dropping my crab traps!!!

Thanks again for your input
Cheers,
Jeff.

Well I think I am as confident as any amateur mechanic could be that there are NO air leaks. I say this because of two reasons: When I bought this thing a few weeks back...I drove it home...almost a 45 minute trek...without having to pump the bulb. I also have gone through the fuel system with a fine toothed comb...it all appears ok.

I just read that raising the fuel tank above the engine can tell you if there is an air leak. Like if it doesn't stall while fuel is gravity fed then yes...air leak. Does this make sense?

Am I answering my own question here? Do I just need to bite the bullet and buy a new carb?

Ah man...how do I edit/delete posts?

I finally "went through the fuel system with a fine toothed comb" by removing every fuel line hose and cutting 1/4 inch off or so and re-attaching to every part of the fuel system.

Air leak not apparent...ran without stalling for about 15 minutes.

Took it out on the water...same problem as before. No power after 60% throttle while in gear. It's strange because when you hit that sweet spot...she wants to go...but bogs, then wants to go...then bogs...and if I go past that sweet spot...she just bogs...and if I back off she trolls just fine.

At least I'm learning a bit though huh?

Sounds like you fixed something by re-fitting the hoses... You can get a 1-meter length of that hose for a reasonable price, BTW.

It sounds like the mixture is incorrect above 1/2 throttle. That being the case, you may be able to get a clue from the spark plug. Shut down the motor when bogging, and pull the plug. If it is very dry, you are too lean. If it's black, you are too rich. Keep in mind that under ordinary conditions, there will probably be some liquid fuel on it, because you killed the ignition while running. Did you verify the jet numbers? Slow-speed jet #35, main jet #60? In either event, it would appear that you likely have a carb issue... unless you have that rare CD that is not advancing the timing properly. You can check that with a conventional timing light.

Hi Paul,

I'm going from the parts manual that I found online for this motor. Main Nozzle and Main Jet #70, labeled in the manual, come from the bottom of the carb (near the float). There are no markings on these parts when I take them off the carb though. Pilot Jet #42, labeled in the manual, comes off the top of the carb, it has a litte 42 inscribed on it.

My last ditch effort before buying the 6A2 carb will be to take it out on the water and try tightening in on the air/fuel screw.

What is your goto carb dip for the 4 hour soak?

Thank a million!
Jeff.

p.s. Whats a rare CD?

Forgot to say yes...when I replaced the spark plug a few days ago out on the water...the used one was quite black. I had a look at the replacement one last night at it also does show signs of black carbon deposit.

Sorry, I was looking at jet numbers for the 4A2. My error. Around here, almost nobody buys a 5, since the 4/5/6 are the same motor... 4 and 6 hp motors are much more common than 5 hp versions.

The NSF5A2 and 4B use main jet 65, slow jet 38, versus main 75 and slow 45 for the 6A2. (and of course 60/35 for the 4A2)
If you have a 42 slow jet, you may have a 15-plus-year-old motor (a 5A)... or an older carb (which would be OK)... or someone put the wrong jets in an A2 carb (which would make it too rich).

The nozzle only uses a part number, not a jet size.

The parts catalog you linked to is for a motor made around 1999 -- an older 5A, prior to the 5A2 -- which uses a slightly different carb. The 5B series came into production around 2005, and uses main jet 65, slow jet 38. In any event, the very early carb is NLA, so if replacing the carb, you would need to get one for the A2 series.

And no, you cannot just change jets to change horsepower... they must be matched to the venturi "throat" of the carb.

I must caution that the pilot screw (idle mixture) is extremely difficult to set in the field... requires you be in the water, and have a very good ear, and a very sensitive shop tach. If you drilled-out the brass plug that covers it, be sure to set the screw back to where the factory had it, or maybe 1/4 turn richer.

So... verify the exact model motor you have. Is there a sticker with the model and serial number? What color is the cowl? If it's grey, it's an A2 or older... "midnight black" (actually a special blue), is newer.

If the plug is coming up sooty, you may be too rich, which would coincide with the bogging. Of course, you might be having an ignition issue. Also, be 100% sure that the oil is not overfilled... not even a drop high.

Hi Paul,

So a bit of correction on the history of this thing. It's not an 8 year old motor. The guy I bought it from said he bought it about 8 years ago...he was a great guy but a little old so let's add 4 years to his guesstimate. He was the original owner and gave me the original manual, an extra spark plug, and even the receipt of the last time he had it serviced... in 2012 a carb clean and oil change.

The plaque on the motor says: model: NSF5A (3H6-1) serial: 11189. Rated: 3.7 Kw. Full Throttle Range - 4500-5500 RPM.
There is another plaque for the emissions control information and that is stamped 1998.

So I do have the old one...Man I hope I didn't buy a lemon...And no 6hp carb for me in the future huh?

Yes, you have an oldie but goodie. All of the carbs will interchange, so the 6A2 carb is fine. The older 5A carb had shorter passages, so used richer jetting (for the same air/fuel mixture). I would re-check the oil level. They are very sensitive to being over-filled... and all 4-stroke outboards do gain oil.

So assuming the oil level is good...would you say that putting the 6A2 carb is a plug-n-play replacement? Will there be much tuning involved?

Thanks a million for all your help! I wish there was something I could do for you. If you are ever in Victoria B.C. Canada... PM me, I'll take you out for lunch!

Yes. The 6A2 carb is the most common upgrade to the MFS/NSF4/5/6. After installation, you will want to adjust the idle stop, but that should be all. If I'm ever near you, I'll let you know. Maybe we can go for a boat ride.

Hey Paul and everyone.

Just a quick note with an update to my problems...They seem to be solved!! Hip hip horray!

So new pump, new spark plug, new oil (making sure to keep it on the low side), and as a last ditch effort I ended up removing the brass plug that was hiding the fuel/air mixture screw. I certainly wasn't confident as this 15 year old engine has never had the fuel/air mixture screw adjusted before, so why would it now? Anyway the problem of bogging down at full throttle continued, until I warmed the engine for about 15 mins, and started playing with the adjustment screw (I did memorize the factory setting number of turns just in case I got in over my head).

Well anyway, finally, I found the sweet spot and was cruising around with no bogging down at about 15 knots at full power...(downloaded GPS speedometer) what a great feeling!!!

One question though. I found that this engine would take, sometimes, 10-20 seconds to actually get up to top speed/full rpm once I'd applied the full throttle. Is this normal? Anyone else's boat that I've driven in the past, (usually much higher HP though), when you give it throttle, it responds with power almost immediately.

Could this, as well as the need to adjust the fuel/air mixture screw, be preliminary evidence that the carb is on it's way out? I mean the carb is 15 years old and has probably been cleaned many times...could there be enough scratches, or irregularities in the jets of the carb that could be contributing to this behaviour?

Possible. There is always some lag, especially in 4-stroke motors, and likewise in small displacements. For now, enjoy !

ABSOLUTELY!!!

Happy boating to all!!!