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

[Sea Rider]

Any brand OB will not perform same when at 6 K feet of altitude, OB's are factory delivered tunned to work at sea level. Check if there's a high altitude kit for that OB, should work much better along a prop maximization with one less prop pitch. Install a new spark plug well gapped to specs, see what happens. Have your tried that OB at sea level ? Which gasoline octane are you using ?

Happy Boating

 

[pvanv]

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. Carb spray by itself is not aggressive enough to clean the small passages.

If less than 4 years old, have your dealer check it out under warranty.

Luis, Why do you think the motor is at a high elevation?

 

[ondarvr]

In one of the later posts he mentions being at 6000 feet.

 

[Sea Rider]

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.

Just to know, was that OB tested at sea level prior being taken to final 6 K boating elevation ? did it performed nicely ? if so, probably high elevation operation is somehow affecting its performance at 3/4 to full grip. At higher altitudes need less diam carb jets to compensate poor carb oxigenation and too much fuel intake into combustion chamber. Probably at that throttle range OB is kind of drowning in fuel which will never be burned adequately, just my 0.5 cent theory.

Read somewehere that you should re-jet your beloved OB starting 1 K feet of elevation & up.

Happy Boating

 

[pvanv]

Sorry, I missed the altitude note. Indeed there is an altitude kit (both jets) for the MFS6. I would call Tohatsu America directly at 214 420 6440 with your exact model (MFS6A, A2, B or C) and serial, as well as your elevation, and they should be able to help you select the best kit. Of note is that usually, the too-rich condition of high elevation typically does not cause a motor to stall, but rather foul plugs and make less power. The jet kits for the MFS6 are usually around $21.00 or so. Even if you do end up needing a new carb, you will probably want the smaller jets at 6K feet.

 

[Sea Rider]

If plan using that OB at 6K of elevation at all times, go for the jet kit, bear in mind that will need to re-jet OB when used at sea level with standard delivered jets. Once with jet kit installed, install an inductioin tach, go for a wot spin lightly loaded and check if OB runs at near 6K revs, if too low could maximize a one less pitch to pull revs up for combo to perform much better throughout the whole throttle range.

At altitude OB needs least fuel octane use, re-jet and prop maximization to get all those horses out of that 6 HP.

Happy Boating

 

[DMcK]

I believe the altitude is not the primary problem. It may be a secondary problem, and I've ordered a #72 main jet as suggested by Tohatsu America for use when I get this problem properly sorted out.
The motor is already running a low pitch prop (it's for pushing a sailboat), and it's not practical to test it at sea level given that I live in Colorado...

In any case, this motor has run ok at this altitude in the past and even if it was running a bit rich it shouldn't lead to the extreme nature of this hesitation/dying. I really think I'll have to soak the carb in something fierce or maybe just replace it. If I was paying shop rates, I'm sure it would be cheaper to just replace it but I hate to be defeated by something simple like this...
Doug

 

[pvanv]

If you get a gallon can of real carb dip, and do a 4-hour soak in it, followed by a blow-out with spray, than may melt out an organic varnishing in a passage. Otherwise, you may need to try a new carb.

 

[DMcK]

Well, finally, after all this time, I believe I have it fixed. (I did do a long carb soak, but that didn't change anything.)

I replaced the spark plug and the problem seems to be gone. The motor starts easily, as before, but now will run under full load, full throttle, for as long as I want.

What I think may have been happening is that under certain load conditions the plug may have exhibited an internal short and then recovered on cooling. This would not have shown up with the in-line spark tester and would not have shown up when looking for a spark with the plug external to the cylinder.

I now have some spare carb parts, but at least I didn't spring for a whole new carb! Hopefully this long saga will help someone else.

Regards
Doug

 

[Sea Rider]

Yep, most times plugs are overlooked and are so darn cheap....

Happy Boating

 

[Millihelen]

Doug (DmcK),
Thank you for your post. I was having the same issue as you and sadly had put money into parts and still same symptoms. I was about to take the outboard to the shop when I found your post. I replaced the spark plug and sure enough, that fixed it. Since I cleaned out my carborator so many times now, the motor starts right up.

 

[Sea Rider]

If OB runs spotless at idle, fast idle or any other throlle setting on muffs while geared with no miss, carb should be all right. Seems an electrical cond, misses when only at load. Probably CD/IC it's on its way to become completely shot. One strory is running an OB at load on muffs, other story is running same at full load while pushing a boat.

Happy Boating

 

[bruceb58]

Running on muffs with no load means absolutely nothing. The engine has to be under load to diagnose a carb problem at other than idle speeds.

 

[Sea Rider]

The only way to test an OB at max wot rpm & load is with a special test prop, OB must run on a proper water container while geared to check if produces max wot rpm range.

Most times plugs, fuel pump and electrical components are overlooked, all is blamed to carb issues. If carb is cleaned to immaculate perfection should not be cleaned again, it?s not good fingering a carb repetedly, a complete carb kit change won't cure the OB if being plug, fuel pump or electrical related.

Discard symptoms testing with new plugs. When OB misses at 3/4 to full throttle, if springs back to life when priming fuel bulb, you have a fuel pump condition, check fuel pump diaphragm cond.

Happy Boating

 

[bruceb58]

The only way to test an OB at max wot rpm & load is with a special test prop, OB must run on a proper water container while geared to check if produces max wot rpm range.

Of course you didn't say that in your previous post.

 

[Sea Rider]

Running on muffs with no load means absolutely nothing. The engine has to be under load to diagnose a carb problem at other than idle speeds.

Sorry mate, what I wrote was "If OB runs spotless at idle, fast idle or any other throttle setting on muffs while geared with no miss, carb should be OK. Geared means its working under load. At load can troubleshoot carb and all electrical parts as well. If carb has been well cleaned, it's not a carb issue...

Happy Boating

 

[bruceb58]

How do you have it geared with a load while on muffin?

 

[Sea Rider]

Sorry will stop further posting as seems will be inevitably be having endless discussion on tthe matter, which is bad for the majesty of the Iboats Forum. Don't want a second E fuel endless discussion episode here, just on a different matter.

Happy Boating

 

[forever_BAD]

Thank you Doug (DmcK)! I changed the fuel pump, thoroughly cleaned my carburetor, and did a couple other things and still kept experiencing the issue you described in the initial post. Thankfully I read the fix you posted and changed the spark plug and that fixed everything. My 2018 6hp 4 stroke Merc is purring like a kitten and running real smooth. I bought it at a nice price and the guy even did a small demonstration to show it running.(Less than a few minutes) For a minute I thought I'd been had, thankfully it was a ~$3.80 fix. Thanks again.