Merc 6hp 4 stroke fouls plug at high power

I have a Merc 6hp 4strokec outboard motor that I use as a kicker motor for my 24’ sailboat. This motor will run all day long or I should say all night at low to mid power settings. I have done it on multiple occasions. The problem lies when I decide to use a high power setting if I’m pushing into a head wind or against a strong current. Without fail , the plug goes bad, usually in minutes or less, and the engine refuses to run until a new plug is installed. Obviously, a bad situation as high power settings imply the definite need for power to manage a situation. I talked to a Mercury tech and he basically gave me the Coast Gaurd salute. Sorry coasties, your service is appreciated.

So, I have never repropped the motor and I am wondering if I’m loading the engine up at higher power settings, but not attaining good, healthy engine rpm. If the motor is propped for a dinghy, would the motor operate at a higher rpm because of a higher speed through the water, less load(?). A sailboat, on the other hand, has a limited top speed, so the motor will be more highly loaded and maybe not be turning at an optimum rpm for the power. Would this cause plug fouling?

If so, suggestions on prop pitch would be appreciated.

if not, ideas on the actual problem would also be appreciated. Thanks.

 

I'd get a lower pitch prop, it's cheap, quick, and can be used as a spare if it isn't correct.

But also check to see if something made a home in some portion of the intake tract. This could allow it to run fine at lower rpms but restrict the air at full throttle.

 

Make sure the choke is completely disengaging.

The levers in modern small outboards are plastic and soft and the actuating rod sometimes hogs out the lever and the choke flap stays slightly closed even though the knob is pushed all the way in.

When this happens, the motor will start and run fine for a while, longer at slower speeds shorter at higher, until it starts running like crap, cuts out and won’t restart due to plug fouling.

Also make sure you have the correct plug and not a plug someone bought at the hardware store in a pinch that’s for a lawnmower.

 

Hi LP, after checking that your choke butterfly does actually open all the way when fully disengaged, and that there are no air restrictions from dirty filter medium, dislodged baffles, or eg. a wasp nest, if you still have the same issue, check your main power jet in the carby. It may have been replaced with a bigger one, or even been drilled out bigger, as speed freak tinkerers sometimes do. The mechanic should have checked this. Another thought, is it blowing smoke, like is common with worn rings or valve oil seals; and is it using the correct engine oil viscosity.
On 2 strokes I always use iridium spark plugs, they resist fouling much better, and may help in your situation with a 4 stroke, except they cost twice as much, or more, and you may need a few.

 

Dry carbon fouling or shiny oil?

 

Send a picture of the plug showing the insulator and both electrodes, in color and close up

Run the following tests on a defective plug.

Google Search https://www.google.com/search?q=how+to+test+a+spark+plug+with+a+multimeter&oq=how+to+test+a+spark+plug&gs_lcrp=EgZjaHJvbWUqBwgBEAAYgAQyCQgAEEUYORiABDIHCAEQABiABDIHCAIQABiABDIHCAMQABiABDIHCAQQABiABDIHCAUQABiABDIHCAYQABiABDIHCAcQABiABDIHCAgQABiABDIHCAkQABiABNIBCDk5NTlqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8


Also, is the engine getting up to the operating RPM when under max load

 

Thanks for all of the input.

I will check the choke operation to make sure it is fully open.

I am reasonable sure the main jet is good, but won’t rule it out. I bought the motor from a guy that only used the motor to push his sailboat into his lift and I feel like he bought it new.

The plugs that are fouled don’t appear to have any carbon or oil. They are not obviously fouled. I’m not sure if I still have a fouled one to ohm for internal resistor issues.

The plug is proper. After the first one fouled, I bought replacements per mercury specs.

I can’t say for sure if the motor is getting up to full rpm. I think it is producing full power, but can’t say if it in the right rpm range. I could be lugging the engine due to improper prop pitch for its usage.

I’ll check the filter medium while I’m looking at things. I may have launch and try to foul a plug on purpose.

 

There is no filter medium on that motor, just a coarse wire mesh to stop things like insects getting in.

I did have the original spark plug separate at the insulator. Something I've never seen before. I am having a hard time seeing how your plugs are fouled if there is no visible deposition...

Have you tried running it with fresh, alcohol free gas?

My Merc 6 has started behaving similarly after about a year of use on my tender. It will no longer rev up to the maximum and allow me to plane the dinghy.

I assumed it was internal carb related. After taking the main jet out and cleaning it, checking float height and function and cleaning out all the other passages with spray cleaner, it runs marginally better, but still not as before.

I suspect that it may be fuel related. The last couple of tankfulls have come from a storage tank with gas several months old. I did use stabilizer but gas nowadays doesn't seem to hold up at all well and tend to leave a varnish like deposit.

I have read the best solution is to soak the carb in a can of carb cleaner to dissolve any deposits.

 

The number one overlooked component during carb cleaning and rebuilds and the one often responsible for being down on full power and top speed is the emulsion tube (different manufacturers call it different names) which is located above the main jet and part of the main “stack”.

Over time, the main jet often sucks crud up from the bowl fouling the small holes in the tube.

When you take the carb apart, remove the main jet, then press the main tube down from inside the throat of the carb to get the stack to fall out so it can be thoroughly cleaned.

If/when the stack is crudded up, simply spraying and soaking a carb isn’t good enough, the tube really needs to be removed and thoroughly cleaned to ensure that every small hole is clear because they all play an important role in properly atomizing the fuel.

If the fuel isn’t getting proper atomization, the motor often won’t make good power or reach full RPM.

Regarding the OP’s motor issues, I too am unclear how a plug can be fouled if it doesn’t have carbon or oil deposits.

Plug “reading” is one of the best ways to pinpoint issues.

Post a photo of what the plug looks like.

Perhaps the issue isn’t fuel related, but spark in that your coil heats up and is breaking down and by the time you replace the plug it has cooled and works well again, although typically there would be some indication of this from a plug reading.

 

he said plug goes bad, not fouls

I assumed he meant fouled..

 

Read it again.

OP asks if the motor is loading up, could that be fouling the plug, and whether changing props may be a solution.

What can break down under load is spark due to a weak ignition caused by low voltage, bad grounds, bad coil, bad ignition wire, bad module, etc..

A Tohatsu/Nissan/Merc 6 is about as simple a motor as you can get these days and pretty easy to troubleshoot if you are methodical about it and pay attention to its pattern of behavior.

Parts are cheap enough that you can just take your best guess and throw money at it, but I like to know what exactly is the root cause so I can have faith that the motor will work when I need it to.

 

One additional thought that may or may not be easy - can you drop the thing on a tender and run it around at 15 knots for a while and see if it still behaves the same way?

I seem to remember they used to require a timing change after break-in. I've found plenty of twenty-year-old motors that had never been timed over the years. And the timing for a kicker would be a bit different than for a tender.

Amita 3 - Mercury/Tohatsu (7.8 x 7") Solas RH Propeller, 5011-078-07 https://boatpropwarehouse.com/products/amita-3-mercury-tohatsu-7-8-x-7-solas-rh-propeller-5011-078-07?_pos=1&_fid=6d48a0f0f&_ss=c

 

Oil level or ignition coil perhaps? The request for photos of fouled plugs is important for diagnosis, but you're stretching the limits of an online forum with this one.

Year model details are critical also, as particular models have strengths and weaknesses, and Merc does a lot of rebadging, mostly Tohatsu these days.

 

I have just landed back here accidentally and read your post properly this time, apologies for that.

So she runs fine until the load goes through the roof when you need full throttle to get anywhere.

Most outboards have spark timing indexed to throttle position, in fact on most 2 strokes the last ~⅓ of throttle movement was all timing, carburettor butterflies were already fully open for that ⅓ of lever travel (or handle twist).

I foud a link to a PDF service manual which should cover your motor, but I'm working on a phone right now and scanning it is more tedium than I'm up for today. It does seem genuine though, if your motor is Tohatsu.

I would imagine that you have a timing plate under the flywheel that is directly connected to the throttle mechanism, for advancing ignition timing; and this means that at full throttle setting you are getting maximum timing whatever the engine RPM. Overloaded outboards are generally over advanced re ignition timing because of this hangover from when it was the best system available.

This over advancing causes an escalation in combustion temps, leading to a situation called preignition or pinging, to the point that the engine cannot deal with the heat produced and components and/or deposits start to glow red; and failures occur. In a two stroke it is generally seized or holed pistons, in a four stroke it is generally damaged valves.

If I am correct then you have been extremely fortunate as the plug has failed first and shut the motor down. This however does not mean no other damage, and premature failure is possible.

The cheapest way out is to put the lowest pitch propeller you can find on it, as others have alluded to. Don't worry too much about specifics, just get your WOT RPM up as far as you can with the lowest pitch available, you are never going to reach factory limits on a 24 footer unless she comes out in a big sea.

Then, assuming you can find the settings, like if perhaps that manual is engine appropriate, check WOT timing and RPM (a little Chinese battery tacho will be a cheap investment). Most of this requires the lid to be off and possibly the flywheel cover, extreme caution of course. It's a flat water job, particularly if you're the skipper, crew & cabin boy in one.

It may be wise to knock max timing down a bit if she won't reach say 4500rpm. Say it's 27°, bring it down via the correct adjustment to 21°. This is really suck it and see stuff, as the outboard is incorrect for the application and will be whichever prop you fit.

The best solution is to trade it for a Tohatsu Sail Pro 6hp, which is probably the same engine as what you have with a lower unit specifically designed for heavy boats.

I had one for a few months on a Wharram Tiki 30, and it cruised that boat at 5Kn, with a 7Kn top speed. They also have an upgraded charging system for sailboats, and you will know about the economy, even with an over propped motor.

Try the prop change then get back to us if it doesn't fix the spark plug issue would be advice at this point if an upgrade is out of bounds.

Here is the manual I found, not sure if Merc do their own 6hp 4 str, but I know for sure they do the Tohatsu. Model & at least starting serial numbers help a lot. Mercury are dated via serial number.
https://www.tohatsudelar.se/onewebmedia/MFS 6D servm.pdf