Nissan 50HP won't go over 4400rpm

[kaferhaus]

Recently bought this boat knowing that it hadn't been in the water for over 2yrs and the original owner did not drain the carbs.

Initially fuel was pouring out of the bottom carb (3 carb motor). Took all three carbs apart and cleaned them out, no more flooding, motor idles fine and runs fine on the flush hose.

Put it in the water today and all is good until you try to move from 2,000rpm up, then there's a hesitation and the boat accelerates to approx 4400 rpm and no higher. Doesn't "miss" just no power and the midrange "bog".

After returning home I realized that I never did pump up the primer bulb.... but I ran this boat for 4hrs (fishing) at various thottle settings and it never once even tried to quit so I assume the primer bulb is not the issue?

Boat and motor are 2000 models.

I would assume that this is a fuel issue perhaps the pump? I was alone and so could not squeeze the bulb while under throttle to check it that way.

Good spark everywhere and the plugs were new.

This boat and motor have seen very limited use since new as the original owner had a stroke only a few months after buying it. It sat for several years and he eventually got somewhat better and used the boat another 5-6 times before his health went south again. It then sat another 2yrs at least.

Bottom carb was full of varnish, float was stuck etc. All that works fine now.

Upper two carbs were fine but cleaned them anyway. Guess I should have taken the fuel pump (I think it's attached to the bottom carb?) apart too..

The engine looks absolutely new.... in fact still "smells" new..

This engine is supposed to run 5200 - 5850 rpm..

 

[pvanv]

Re: Nissan 50HP won't go over 4400rpm

You may or may not have two separate problems. With a motor of this age, it's prudent to go through any and all of the rubber parts (including the fuel pump, hoses, and even the impeller) just to verify that everything is as it should be.

Are you running premixed fuel, or using the onboard oil mixer pump? A plumbing problem there (at the mixer) could cause a lean condition under some circumstances, which could contribute to both of your problems, as well as cause combustion chamber temperatures that are too high -- check the spark plug coloration after a WOT run to see that they aren't overheating due to a too-lean condition.

The midrange bog could be a fuel delivery issue, but that's not all that common. Still, if the lower carb was full of varnish, it wouldn't hurt to go through the pump (yes, it is on the lower carb.). Did you check/set float levels, etc., in all 3 carbs? It's important that they be setup correctly, and properly synchronized. Also check the ignition timing and synchronization. Slow ignition timing could cause this problem. Also get a new element in the fuel filter, as they can get gunked up, yet still "look" OK.

The high speed issue may be related to the midrange bog, but it could also be a prop situation. There's a whole range of props for this motor, from about 7" through 15" pitch. There was even a 4-blade, high-thrust "barge" prop available. What prop are you running? What kind of boat is this on, and how heavy is it? If you're propped too steep, or if the motor is too deep in the water, you might not get to proper WOT rpm. Are you using a good Tachometer? Some of the cheap tachs are quite inaccurate, so there's even a possibility that your motor is achieving the correct rpm -- working OK, but the tach doesn't read right.

A parts catalog, available at Tohatsu Guru's excellent website: http://www.internetoutboards.com/PartsCatalogMenu.htm can help a lot. Also, getting a Factory service manual from any Tohatsu/Nissan dealer wouldn't hurt. Lots of good information in it -- cheap investment.

 

[kaferhaus]

Re: Nissan 50HP won't go over 4400rpm

I should have provided more information in the first post, sorry.

Plugs look fine.

I'm using the oil injection and it appears to be functioning fine. I ran about 4.5 gals of fuel through it and it used several ounces of oil.

The prop is 13.5 pitch.

The boat is a fiberglass Dawson 1600 flats boat which is a light weight boat... rolled edge laid up fiberglass, not a hull and liner.

Tach is the Factory Nissan tach.

Floats are all set properly and in any event due to the design, float level cannot be adjusted as the needle just slips into a slot that cannot be "bent".

The fuel filter does "look" fine, won't hurt to change it but I seriously doubt that's the problem as the primer bulb has no problem getting fuel through it very quickly indeed.

I don't see how the ignition timing could "get off" on an engine with electronic ignition when none of the components have been changed or altered. The advance mechanisim is timed properly with the carbs.

all the rubber on this engine appears just as it would new... soft and supple, no hard or brittle hoses.

Unfortunately, no Nissan dealer around here I think the closest is about 50-70miles away and they carry virtually no parts stock.

I've ordered the factory repair manual but I don't think that's really going to help me with this problem.

My "gut" tells me this is a fuel delivery problem as far as the high speed issue is concerned.

The hesitation is maybe a separate issue but I think it's related to fuel volume at acceleration. What I don't understand is how does the fuel get "richened" for initial acceleration? I know on cars that had carbs there was an accelerator pump that initially enrichened the mixture, on fuel injection systems the injectors momentarily inject a "burst" of fuel in some systems. How is this done with an outboard motor?

Also on the parts manual for this engine I see a bypass valve in the fuel pump. could this be sticking and causing the upper carbs to not recieve good fuel pressure?

 

[TOHATSU GURU]

Re: Nissan 50HP won't go over 4400rpm

Your closest dealer is in your same town. There are five dealers within 35 miles of you. I would urge you to take the engine into a dealer and have them diagnose the problem.

Stauter Boat Works 251-666-1152

 

[kaferhaus]

Re: Nissan 50HP won't go over 4400rpm

Your closest dealer is in your same town. There are five dealers within 35 miles of you. I would urge you to take the engine into a dealer and have them diagnose the problem.

Stauter Boat Works 251-666-1152

Thanks TG, Stauter boat builders are good folks, but take lots of vacations... they're closed until the 2nd week of Jan.

The other supposed close by dealers are either no longer handling Nissan engines or are closing down.

I'll take someone out with me and have them hit the primer bulb while I'm at full throttle to see if it's the fuel pump. I do remember that when I took the carbs off for cleaning (after just having ran the engine) the hose from the fuel pump that leads to the upper carbs was dry... maybe the check valve in the pump has trash in it?

I can't see this as being that complicated of a problem, thought maybe you of all people had seen it before.

Thanks

 

[TOHATSU GURU]

Re: Nissan 50HP won't go over 4400rpm

Unfortunately the symptom(s) can be caused by a variety of issues...It's like telling your doctor, over the phone, that you have a stomach ache. Lot's of things can cause that. Lot's of things can prevent you from reaching top RPM and cause hesitation. Your first step is determining what system(s) are out of whack. You eliminate ignition, you eliminate mechanical and then and only then you pick a part of your fuel system and have it. Or, you can just start randomly screwing around with parts....You might get lucky and find you have a pin hole leak in a fuel line. You might be unlucky and waste 10 hours, $1000.00 and be no closer to finding the problem. As expensive as a dealer fix can be, it's usually cheaper than shot in the dark backyard mechanical guess work in the long run. Now if you really want do this on your own. Start over again with the carbs. Take them apart, soak them in carb cleaner(not spray can cleaner) and try it again. Ninety percent of all outboard problems are fuel system related and 90% of those are carburetor issues.

 

[kaferhaus]

Re: Nissan 50HP won't go over 4400rpm

Well the fuel pump looks fine... in fact looks brand new just as the motor does...

This boat is rated at 90HP but as I stated equipped with the 50, I cannot find any numbers on the prop except 13.5C cast into the back of one of the blades.... so I don't even know what pitch it is unless that's it. However Nissan doesn't show a 13.5 pitch prop .

According to their site the standard prop is a 11" X 12.4" pitch and that prop is designed for a "light load".

The original owner says nothing was ever changed on the motor, the prop is the factory original. He also says he never ran the boat over about 15mph!! never tried too as he only fished for flounder and where he fished it was a no wake zone most everywhere. He has no idea if the motor ever revved past 2500 or so rpm... He also said that's why he didn't let them sell him a bigger motor because he knew he'd never need it.

So according to Nissan "light load" = 500lbs... there's no way this boat is even close to 500lbs... 16ft hand laid glass with a side console, front and rear casting decks, 3 batteries that have to weigh at least 50lbs each, 2 6 gal cans of fuel ... almost another 100lbs right there, trolling motor.... and then add the passengers!

Add all the other gear like anchors, paddles, electronics, steering gear etc. etc.

Could a prop with way too much pitch for the load cause the "hesitation"? Obviously that could be keeping it from reaching the proper high rpm range it needs to.

Thanks for any ideas.

 

[DJ]

Re: Nissan 50HP won't go over 4400rpm

I think you MAY be overpropped but that's hard to determine, at this point.

Your mid-range "stutter" is not normal and I think a clue to your power issue. I had a Suzuki 140 (carbed) that did the same thing and was notorious (at least mine) for clogging carbs. The "stutter" was my first clue to unzip them again.

I would not trust a fuel pump that suffered the same neglect that the carbs did. Especially with some of todays funky fuels.

If you haven't I would definetely replace the fuel filter. I eventually put a Racor 10 micron on my Zuki and it helped, not to mention judicious amounts of Sea Foam in each tankful.

 

[pvanv]

Re: Nissan 50HP won't go over 4400rpm

I'm in full agreement with TG and DJ. Go through the carbs and pump again. And please do soak the carbs, not just spray them. That would include the pump on the lower carb. Definitely a good idea to change the filter element as well.

The midrange stumble is probably not prop related, so see if you can verify everything (fuel, mechanical, ignition) on the motor one more time. You really want the power curve to be smooth and correct before changing out props.

Too bad the original owner never ran WOT, since now you don't have anything to gauge your performance against. No benchmark. So from the info we have, it's not clear whether the motor is actually a little small for the boat or not.

It does sound like you have a 13" prop, which is likely a bit on the steep side for a heavy load, but prop pitch is always a "test and see" (trial and error) deal. Each boat is different, though, so it's impossible to tell at this point. Once you correct the stumble, then you can check out the effect of different props. Our local prop shop will inspect a prop to give you the low-down on it (precise diameter x pitch, overall condition of hub, condition of balance, etc.). If they have a used prop for your setup, they will sometimes loan it to you to try a different pitch. Your local prop shop may or may not be able to do this, but it never hurts to ask.

 

[kaferhaus]

Re: Nissan 50HP won't go over 4400rpm

Thanks Guys!

I did take the carbs back off and soaked them for 4 hours in brand new carb cleaner. Obviously I don't have itty bitty microscope but all the passages pass air easily (as they did before)... I should have posted pictures as this entire rig looks like it just rolled off the manufacturers lot...

Fuel filter looked brand new, and you could pour gas right through it... replaced it anyway.

The engine runs fine on the hose, no problems with acceleration, no stumble. but it's not under a load then either..

Carb timing is perfect, ignition timing is perfect, plugs look fine and there's no "misfire" at any time.

Surely it could not be my not pumping the primer bulb up firm? I know I forgot to do that.

Anyway it's hard to tell anything without the boat back in the water under load.

The fuel pump is working fine.... pulled the plugs and ran a line to a jar and spun the engine.... believe me it's pumping plenty of gas!

I still don't understand how the engine gets a "burst" of fuel when you hammer the throttle? Can anyone clue a stupid guy in on that? I would suspect that however that is supposed to happen isn't and that could be the cause of the stumble.

 

[TOHATSU GURU]

Re: Nissan 50HP won't go over 4400rpm

Once the engine is running the fuel pump supplies the need pressure for the fuel so the initial prime bulb lack of being pumped would not make a difference. Your prop is a cupped 13.5....And very high for your application. The engine does not get a burst of fuel...The system is as simple as it looks.

 

[kaferhaus]

Re: Nissan 50HP won't go over 4400rpm

Once the engine is running the fuel pump supplies the need pressure for the fuel so the initial prime bulb lack of being pumped would not make a difference. Your prop is a cupped 13.5....And very high for your application. The engine does not get a burst of fuel...The system is as simple as it looks.

Thanks Elvin... know I've been trying your patience! Figured as much with the enrichment question as I couldn't see how any initial burst of fuel could be delivered.

And coincidentally the service mgr from Stauter came into the shop today (said he came in to let a customer pick up his boat for the weekend) and got my voicemail. Called me awhile ago and after some questions back and forth (mostly about the carbs.... go figure) He thinks the boat is way over propped and that I've checked everything that they would check at this point. And that in his experience if the engine wasn't getting enough fuel at high loads the engine usually "surges" or acts like it's misfiring at full throttle. The fact that the engine is running smoothly at WOT also tends to make him think it's the prop.

He said he's seen them "rarely" so over propped that it will cause a stumble if the throttle is suddenly moved to full as the prop is overloading the motor from the get go... then catches up. His theory is that Dawson neglected to change the prop when the original buyer of the boat selected the 50HP. But he also said that the prop on the motor was not the "standard" pitch prop that came on it... Dawson may have taken it off of a smaller boat on the lot and just reinstalled it on my boat. His "best guess" is I need to drop about 4 numbers based upon the fact that I've only been out on the boat alone (weight).. says that will likely get me into the middle of the proper operating range with two adults on board.

He was also kind enough to remind me that they are several miles from any body of water and the only way they could "load test" the boat would be to haul it to the river and test it... which they'd have to do from the get go. They get a paltry $150 for that.... LOL. When they come back from vacation he said he'd see if he can find a prop that I can go test. If that works I buy the prop, if not I leave the boat there and they try to find the problem.

When I told him about the previous owner never even attempting to run the boat WOT (which I found odd) he just laughed and said "we de-carbon motors everyday which have been run the same way for years"

Apparently there are folks out there who just putt to their fishing hole and putt back to the ramp...

I'll let everyone know how it works out.

 

[TOHATSU GURU]

Re: Nissan 50HP won't go over 4400rpm

The simplest explanation is often the correct explanation...But not always You stand to gain around 1100 RPM based on the pitch and the cup in the blades dropping down to a 9 pitch....I think that is actually too low based on nothing but my ESP powers. I would drop down to a 10 pitch as a probable compromise on speed and hole shot, but Tohatsu doesn't offer a 10 So I think you will need to make a choice between a 9 and a 11. If you can live with the hole shot I think the 11 is better...Probable 700 RPM increase. If you need the hole shot the 9 will give it to you, but your top speed is going to drop a bit. Propping is an art and the perfect prop can vary on any given day so skip perfection and go with major improvement. If Stauter has a prop you can borrow in that range you need to thank them as many dealers have given up on the "borrow" philosophy of customer service. And you haven't wasted your time on cleaning the carbs....I'm sure they needed it or would have needed it shortly.

 

[kaferhaus]

Re: Nissan 50HP won't go over 4400rpm

And you haven't wasted your time on cleaning the carbs....I'm sure they needed it or would have needed it shortly.

Oh, the bottom carb absolutely had to be cleaned the main jet was plugged, float needle was stuck etc. And while I cleaned all three of them (twice... LOL) the other two looked brand new inside and out from the get go.

However, I cleaned the bottom carb before I ever put the boat in the water the first time as it was pouring gas on the hose (and smoked up the whole neighborhood)

Nissan does show this prop which is a 9.9 pitch 36164-1090M, do you not carry that one?

 

[TOHATSU GURU]

Re: Nissan 50HP won't go over 4400rpm

We do carry it, but for reasons that I can't explain it doesn't pop up as fitting the 40D/50D frame. It does though. I'll have it added to Tohatsu's website as fitting it as they don't show it under their selection chart for your model. We have now added it to our prop department under your size engine.

 

[kaferhaus]

Re: Nissan 50HP won't go over 4400rpm

Service manager called me back awhile ago and said no go on the prop, he didn't have one anywhere near the right size. So I ordered one.

Thanks

 

[Wee Hooker]

Re: Nissan 50HP won't go over 4400rpm

Here is a "back yard mechanic" test that may help if you suspect fuel delivery still might be an issue due to fuel pump/pressure. Put only a gal or so of fuel in one of your tanks. Squeeze the bulb to prime the bowls then start & run the engine. If the motor bogs down at mid/high RPM's try raising the tank above the top cyl and squeezing the bulb a few times again. (Have someone else drive !) This causes a forced gravity assisted flow into the bowls vs relying on the pump to suck it up. If Rpm's improve (even momentarily,) you have a fuel flow issue. I found a weak fuel pump issue in a like running 72' 9.5 hp Evenrude doing this once.

p.s. Don't overlook replacing your fuel line/bulb. E-10 based fuel can eat up older bulbs fast.
hth

 

[Scupper Guppy]

Re: Nissan 50HP won't go over 4400rpm

By the year 2000, had Nissan/ Tohatsu corrected their crankshaft spinning problem on the 40D or 50D 3 cyl outboard? Just curious...

 

[TOHATSU GURU]

Re: Nissan 50HP won't go over 4400rpm

As they have never had a problem with spinning crankshafts your question is incorrectly phrased. Did you mean that if someone exceeds the recommended top RPM by an additional 1500 RPM can they still spin? The answer to that is yes...Just like any other multi-pieced crankshaft you can spin one from abuse or accident. That's why most racers, that run the 50, weld their Yamaha and Tohatsu engines crankshafts prior to competition.