Nissan/Tohatsu 6HP 4 stroke hesitates and/or dies under full throttle

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DMcK

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Hello,
I have a Nissan 6HP four stroke on a sailboat, with the low-pitch high-thrust prop. This motor hasn't been used for a few years, but starts fine and runs fine as long as I don't run it at full or, say, 2/3 throttle. If I do that, it sometimes will hesitate, stutter, and maybe die.
It will always start up again.
It feels like fuel starvation. I think the fuel pump is ok because it seems to pump quite vigorously when I pull off the pipe to the carb. Also, squeezing the bulb on the fuel line doesn't help when it is hesitating, but sometimes a bit of choke will.

What are the most likely causes? Is it possible that a little bit of crud in the float chamber gets sucked up into the main jet when the motor is needing a lot of fuel?

I think my next step is to dismantle and clean the carb, but I don't want to do that on the boat because of the 100% certainty that I'd drop something overboard...

Suggestions?
Doug
 

pvanv

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You are probably right. Could be a thermal electronic problem, but more likely you need to clean the carb. Might have a restriction in the inlet needle/seat, or a floating bit of debris in a carb passage.
 

pvanv

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Also verify the oil is not over full. Between the hash marks on the stick only.
 

DMcK

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Still struggling with this... I cleaned the carb thoroughly (I actually wasn't visibly dirty, but I did a careful job anyway). I have successfully cleaned many small engine carbs in the past, so there's a decent chance I did it ok.

No change. It still starts easily, and sometimes runs well, and often will struggle and hesitate and cut out under throttle. Again, it always starts easily.

I wondered if it was the fuel filter so replaced that. No change.

If it was an electrical problem, I would tend to think it might not start so easily after it cuts out?

What about the fuel pump? It has very low operating hours, but is almost 10 years old. I has sat a lot. Could there be a problem with a pump diaphram?

Oil level is ok.

Thanks for all input.

Doug
 
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pvanv

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Still sounds like starvation. Perhaps the tank vent is restricted, or you could have a slight leak in an o-ring in the primer hose.
 

DMcK

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I agree that it sounds like fuel starvation.
It's not the vent. One if the first things I tried was to run with the tank cap completely off. No difference.
I don't think the hose is sucking air through a leak, because I can see fuel in the filter. But, it is true that I haven't replaced the hose (yet!).

Doug
 

DMcK

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This is getting frustrating. I replaced the fuel filter, and thought that had done it, because for the next 10 minutes it ran perfectly. Next time I took the boat out, the problem came back...
Yesterday, it was really bad. It would hardly run above idle. When I pulled the fuel pipe off the carb, the pump was producing loads of fuel and plenty of pressure (fues sprayed everywhere as I was pushing the pipe back on).

Float valve (again?). When I clean the carb, that seemed perfect. This is a *very* low hours engine.

Is there any kind of electronic problem that could be causing this? Personally, I doubt it. It seems like starvation, but I can't find the cause....

Doug
 

Sea Rider

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When engine dies, how fast is it to start again, would say instantly, takes some seconds ? could be electrical related, component heating and shorting soon after or moisture issue. Are both plug's sparks strong ? Clean all electrical grounds to be immaculate shinny including powerhead contact points, probably sulfated ? Was the OB stored indoor/outdoor when sitting for years ?

Have you checked fuel pump's internal diaphragms ? those are quite thin and E-fuels are not friendly with them, will end cracking, ballooning them if OB was stored with fuel inside pump for extended time periods.

Happy Boating
 

DMcK

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If I close the throttle down to a lower speed, it will immediately run fine. If I open the throttle more, it will stall out. If I then close the throttle down to a normal starting position it will start right up, maybe with a couple of pulls. So, it will start in a few seconds, less than 10, often less than 3.
The motor was stored indoors and looks completely immaculate under the cover. It really is nice, shiny, and in "showroom" condition. It has never been near salt water. I haven't checked the plug because it starts so easily. I do feel it is more like a starvation thing, but the fuel pump seems really strong. I haven't dismantled the fuel pump, but when I pulled the pipe off the carb, and then was replacing it while the motor was idling, it really was spraying fuel all over the place. It seemed that it could produce both volume and pressure...

I think my only option is (another) carb tear down...

Doug
 

DMcK

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Well, it's the new season and I *still* have this problem. Today I replaced the float valve, and it's clip/lever thingy just in case it was sticking, (and the float). The existing one seemed just fine so, not surprisingly, this didn't fix anything.
I pulled the crankcase breather tube off, just on the remote chance that there was too much oil in the crankcase and some was getting splashed up at high RPM. No problems, and indeed the oil level is correct.
As usual, it starts really easily and runs fine at low RPM. It seems to run pretty well at high RPM under no load, but if I put it in gear and open up the throttle it will run fine for a period of time (maybe 5 seconds today) and then hesitate and probably cut out. It will start up right away. Maddening!

The internals of the fuel pump seem fine, and it seems to work very well. Plenty of pressure and volume.

Could this possibly be electrical? (Seems more like starvation, but I'm at a loss on that.) What could cause the spark to fail under these circumstances??

Any thoughts are greatly appreciated!
Doug
 

ondarvr

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Put a timing light on it and check the spark when it acts up.
 

DMcK

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Good idea. I haven't owned a timing light for decades, so I've ordered up one of these in-line spark tester thingys. Should be interesting.
 

DMcK

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Well, I tested the ignition system with the in-line spark tester and the result was completely clear. The spark is strong even when the motor is struggling and dying. Unless something really perverse is happening to ignition timing it must be fuel supply/carburetor.
The fuel pump seems to be able to provide lots of volume and pressure, so it must be the carburetor, but what?...

I think there's no option but to do another complete cleaning and hope that there's some small movable blockage that moves around depending on the conditions of fuel flow. Perhaps I'll get it this time. I think I did a pretty good job of that last time around, but what else can it be??
Doug
 

ondarvr

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It is very common to do what seems like very thorough cleaning of the carb, only to have it act up almost immediately, the second or third cleaning usually does it.
 

DMcK

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Another very very thorough carb clean today. Again the motor starts immediately. First pull, in fact. Runs quite happily, even wide open throttle out of gear seems ok. (I wouldn't normally treat an engine as rudely as that).
Again, the problem is when I put it in gear and open up the throttle. After a few seconds it begins to hesitate and will die unless I throttle down. Sometimes it will spit back through the carb, as it comes to a stop.
I'm at a loss. The carb is spotless, I've blasted carb cleaner through the jets and every passage in the carb that I can find.
I could buy a new carb, I suppose, but that seems like it shouldn't be necessary. One thing that occurs to me today is that I'm operating at about 6000 feet, but I don't see why running a little rich would cause this.
What else could I be missing? This is driving me nuts. I've never been beaten by a small motor before....
Doug
 

pvanv

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You may have a teeny bit of metal sprew in a carb passage that will not dissolve, and floats around, getting sucked into a position to block off fuel when throttling up. I would definitely try another carb. What method did you use to clean the carb? Aerosol spray is not aggressive enough. A thorough cleaning will require a complete disassembly of all bits, including the jets and emulsion tube nozzle, than a 4-hour soak in real carb dip, followed by a very liberal blow-out with generic carb spray.
 

DMcK

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I disassembled the carb, as much as I could, including jets and emulsion tube, and blasted aerosol carb cleaner spray through the passages. I soaked the jets in cleaner and blasted through them too.
Unfortunately, to try another carb I'll have to buy a new one which is kind of irritating... It may come to that, though.
Doug
 

ondarvr

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When it's dying try putting your hand over the carb to slightly restrict the air, if it runs better you're still running lean.
 

DMcK

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I've tried giving it some choke when this is happening. It doesn't seem to help. If there's a serious restriction in the fuel supply somewhere, then I guess even the choke can't get more fuel in there...
What I'm failing to understand is where the restriction could be. The float chamber valve is new, and moves freely. The passage to the float chamber seems to have lots of flow.
There's no crud in the bowl. It's spotless. The main jet and emulsion tube seem completely clean too. That's the main supply, right? Everything else is not so important for open throttle operation, right?
It's a pretty simple system...
 

ondarvr

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If using the choke did nothing, then you may need look in a different direction.
 
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