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I have a 1998 26X with a 50 hp Tohatsu Engine. Last year when I would accelerate to maximum, the boat would take off quickly but at about nine mph it would just not go any faster and kind of accelerate and then bog down, want to accelerate and bog down. I know in a car, sometimes this is caused by a bad coil. Don't know about this engine, though, what might cause this. I have tried with both empty ballast and full ballast and it seems it is about the same. I checked speed both ways with GPS and seems to be right at about 9 mph. Appreciate any help in this matter.

Rick

What revs is it doing and check the prop size.

Hello,
Sounds like fuel supply or missfire to me. Try pumping the fuel line While it's exhibiting the behavior to test that system. For ignition make sure the plugs are good. If a cylinder is cutting out, the power changes are sudden / sharp. If it's the fuel the power changes are softer like you are moving the throttle lever.

Good luck

This is pretty much just what I have going wrong with my 40HP 2-Stroke Mercury.

Last two times out I had a quick/sudden drop off from 5100 (normal full throttle) to 4000 RPM. It was suggested to me that one of my "control modules" (one per cylinder which he told me was just like a coil) could be shutting down when the motor gets "hot"...thus that spark plug quits firing and I am down to two cylinders.

But I am going to check for fuel issues first (SeaFoam and fuel filter clean or replace) and then get my carb serviced. Then if I still have that problem I am going to probably have to take it to a dealer.

Good luck.

Cheers,
Jim
Sailing on König

It sounds more like fuel than coils. The coils are tough to diagnose other than buying one replacement, then swapping it around until the problem stops. That's a 200 USD test, but that still cheaper than shop rates to drive it through the door

If we had 10 Mercury FIJ owners on this board, we could buy the fancy diagnostic tool and make this easier....

you dont give much information but also try a new tank to motor fuel line, a pin hole air leak will cause your symptom,i usually just buy a new one every three years and always carry a spare.also what rpm is this at.what year motor, the tldi 50hp has a ecm that stores codes same as late model cars and with your key switch it will tell you what engine system is malfunctioning.

Hello,
Ignition troubleshoot:
If you suspect one cylinder is cutting out, use a timing light to diagnose. Move the pickup clamp to each wire while the motor exhibits fault. If it is a thermally reactive problem it will miss even at low throttle settings after it heats up. Make sure your plugs are in good shape or new just to eliminate them as a possible cause.
Fuel troubleshoot:
If it idles ok and misses under load then you may have water in the carbs (main jet is physically lower in the fuel bowl) or low fuel supply, air getting into the suction / supply side (tank, hose, bulb, fittings) of the engine. You can fit a section of clear hose in the fuel supply hose to check for air intrusion.
Under the engine cover, the filter may be plugged or the fuel pump may be defective. The fuel pump is a simple rubber diaphragm driven by the lower engine case pressure pulses. To check for water in the carbs there are drain screws in the side of each float bowl. Loosen the screw (not all the way out) and capture the fuel coming out into a small jar. Pump the hose bulb if you need to "flush" dirt or water out. Check to see if there is any water in the fuel.

Good luck

Thanks for all the advice to everyone who responded. I will troubleshoot according to your comments, which are very helpful.

Rick

Don,

That suggestion about using a timing light. Can that be done running the motor in neutral with the hose/ear muffs with the boat on the trailer or will there need to be a load on the motor?

Thanks,
Jim

It will need to be loaded to make it miss, is my guess,

The other problem you will have is that the coils dont have wires running to the plugs, you might have trouble getting the light to trigger on the plug firing...its not your grandfathers Oldsmobile after all.

I had the same problem (only WOT) on my old Nissan 30. Nissan mechanic did the check up and could not repeat the issue in the shop (and I am sure it was because in the shop the motor was not pushing Mac on WOT. When I launched again and issue repeated. Out of curiosity I changed spark plugs (I always carry extra set of new plugs) and it never repeated again. I just throw the old ones and get a new spare set. About year later issue repeated this time even on the lower speed with the engine sometimes shutting off. I removed the motor cover to check the engine, started it and works great, put the cover back it shuts off. After frustration I examined possibilities and found that one of the cables was arching and that was causing the issue. I just re-rutted the cable further from the engine cover and it worked flawlessly after that.
So check these two things and you might get closer. I am not sure that the first issue was not the beginning of arching and while I changed the spark plugs I unknowingly moved cable that was arching further from the engine cover.

So check these two things, it might work

Hello,
The Tohatsu does have plug wires. And yes, you can test it with the "muffs" on however it will be hard to hear the miss with the erratic 2 stroke idle. You can make it easier by speeding up the motor using the throttle linkage so as NOT to advance the timing. Check during throttle opening so it's under load.

Zoran: Then ... its past time to replace those plug wires...

Zoran wrote: I removed the motor cover to check the engine, started it and works great, put the cover back it shuts off. After frustration I examined possibilities and found that one of the cables was arching and that was causing the issue. I just re-rutted the cable further from the engine cover and it worked flawlessly after that.
So check these two things and you might get closer. I am not sure that the first issue was not the beginning of arching and while I changed the spark plugs I unknowingly moved cable that was arching further from the engine cover.

So check these two things, it might work

lol.. I actualy replaced the engine and now i have new Tohatsu 50 TLDI pushing my Mac

Rick - misfiring problem on another thread solved with new spark plugs.

Cheap, DIY, and if it doesnt solve it you now have a spare set of plugs.