Outboard mystery problem ? ? ? ? ? ? ? ?

[gyroplanes]

One day my son asked me why I wasn't annoyed at the Tohatsu 50 (1998, carbs model) quitting at full power.

I never had the motor quit. I never run it at full throttle I guess.

I haven't pinned down the exact RPM yet, but at something over 4800 RPM, after a few minutes at this setting, the engine flat out quits. Every time.

It is just as if the key was turned off.

No roughness prior

No slowdown

AND, it restarts every time without hesitation.

Will happily run all day long (I imagine, never did it) at less than 4800 RPM

I checked and cleaned the fuel filter
I checked the fuel flow
I replaced the sparkplugs (annual event for me)

I have called Tohatsu / Nissan dealers....they are all clueless.

All ideas welcomed. My son wants a 90 HP after watching VKM repeatedly on You Tube. I suspected sabotage.

[kadet]

Quitting like that without running rough first sounds like a complete loss of electrical power for the spark to me. If it was fuel based you would expect a bit of rough running before quitting.

On an older motor like that I assume you have a coil in your electrical circuit, this could be breaking down at the higher loads for full revs and cutting out.
Probably needs a tank test with mechanic to truly identify what's going on.

[133bhp]

+1, no symptoms sounds electrical to me. although on my motorbikes, coils normally severly miss at high revs or when hot through expansion of a crack , but I'd try coil first then the the pickup coil, else look at the kill circuit is not being induced by vibes somehow? tie wrap the cutout switch?

keep informed - I've the same engine!

[Capt Smitty]

Attach a timing light and watch the light as the engine quits. If light stops flashing at the same moment the engine quits, you have an ignition problem. If light continues to flash as the engine winds down, look at the fuel system. This method will also show you a misfire, look for erratic flashing light. Sounds like fuel system to me, if it will run all day just below that kill rpm. A squirt of carb cleaner into air intake as it quits may confirm this. Be careful you don't fall overboard. If the carbs stop feeding fuel to the engine suddenly, it will just stop. If it does prove to be fuel related, unlikely a carb issue, I'd look into fuel delivery, from the tank pick-up to the carbs.

[Tomfoolery]

I'd look into the overspeed cutout, if it has one, and the overspeed cutout when out of gear (lower speed than in gear), if it has that too.

My BF50 Honda will cut out and die well below redline when in neutral, using the smaller throttle/choke lever, and it has to be restarted when that happens. I'd have to check the service manual (not on this machine) to see how it works, but it does, so it's something to look into at least.

[Freedom77]

Tighten the treadmill and feed the squirrel some speed Seriously, sounds like an electrical problem. Fair winds and full sails.

[gyroplanes]

Sorry I haven't posted back sooner. I haven't been able to go to the boat since I posted this.

I appreciate al the replies.

I plan to use a neon test light on the plugs next time out. (or the timing light, if it's daylight) i believe (90%) that this will be electrical in nature.

I might do a full throttle run at the slip if it isn't electrical.

I did check fuel flow, it was "OK". I would be shocked if all 3 carbs ran out of fuel at exactly the same moment. The instant restart also kinda rues out that probability (in my mind)

I will keep you all up-to-speed.

The good news. My boat made it's first long passage last weekend. My friend, John, took the boat to Chicago for a lakeside concert. I believe it was an 11 hour passage from Hammond Indiana (1 hour by car). John had to nearly cross the lake to tack in to port. He loved every minute (so he says)

[Don T]

Hello,
There are 3 coils so the probability of all 3 failing together is remote. There is an over-temp kill switch that could be at fault.

[Crikey]

I also think the overspeed kill system is the culprit. When you say 4800rpm, is it exactly the same rpm each time? Not knowing the engine, the three coils could be also linked by a ballast load resistor which heats up proportional to the total current flowing through it, then when it reaches a critical level - determined largely by the rpm - suddenly goes intermittent then cools down and functions again. I had a VW beetle that drove me crazy with this problem once.
The difference between these two malfunctions would be if you can induce the rpm limiter right away when the motor is cold, or this only happens after its had time to reach operating temperature.
A carb based fuel symptom would sputter or miss to some extent beforehand - not just quit.

Ross

[tlgibson97]

How is the fuel pressure at full throttle. I have a 96 tohatsu 50 and the last couple times I went out my engine started cutting out and dieing at full throttle. I found out that the primer bulb had no pressure in it. as long as I keep pumping the primer bulb it runs fine.

Since I run regular ethanol gas, it's probably destroying everything in my engine. My engine needs quite a bit of work though, so that's just one more thing to add to the list.

[island808]

Edit>. Does this have a cam sensor?

I'm not all that experienced with outboards, but motorcycle, inboard and automobile experience would blame electrical. The fact that it. It's out all together without warning sure sounds like ignition. Coils are probably powered by the same source. Only times I've had an engine quit dead at high rpms was on 2 occasions. One, the ignition fuse blew, no idea why, never did it again. But if I had a breaker, it would have kept coming back and I'd have had trouble diagnosing. The othe time, a coil wire rubbed through and grounded. (do not wrap extra long wires around the base of a distributor, no matter how soon you plan on making it right). In both those situations, the engine would not try to fire again until fixed. Which was easy once I found the problems.

Fuel usually coughs at least once before dying completely, especially with carbs. There could be a fuel vent problem, tank under vacuum reaches a point where it reaches critical pressure and stops the fuel quickly? Unlikely but an easy diag.

If its an over temp you'd get an alarm right, and it wouldn't start for a couple minutes or would start hard and maybe be under powered.

Do over rev cutoffs kill the engine on outboards? I've never dealt with a lost prop or other high rev situation.

Long shots, as I doubt these engines have any of this stuff. Cam and crank sensors. (quick search shows they might, so That's my new favorite)

Long and short is, has me baffled. Just quits and only at high rpm. I'm interested to hear the resolution.

[Tomfoolery]

Do over rev cutoffs kill the engine on outboards? I've never dealt with a lost prop or other high rev situation.

According to the manual, my Honda BF50 does.

[island808]

Do over rev cutoffs kill the engine on outboards? I've never dealt with a lost prop or other high rev situation.

According to the manual, my Honda BF50 does.

Manuals... I ought to download one of thems.

And it kills it till you hit the starter again you mean yea?

[Tomfoolery]

It did on mine, though I don't remember what the manual says about that other than there is definitely an over-speed protection function that's supposed to activate at 6600 rpm, +/- 200 rpm. I fired it up this spring, and ran the engine up with the neutral throttle lever, and promptly sped it too much (though I wasn't looking at the tach), and it died. Started instantly when I hit the key again, and didn't do it again while playing with it, or while boating after that, but it certainly did do that in the driveway.

It may also be possible that the threshold is lower when using the neutral throttle. I've heard of such things, but can't recall in what context.

Getting older sucks.

[island808]

Modern cars rev limit in the 4000s or below when clutch is depressed or in neutral or park to save the transmission ans engine. I hear that the engine is sloppy when not under load and can spin bearings and in general hammer the journals. otherwise they limit at redline or just beyond. Diffent scenario though. They limit not ignition kill. And I don't remember them doing that back in the 90s.

Makes sense that boats would do it, should be in a situation where you overrev. No missed shifts when boating and spinning the tires probably isn't a valid way of keeping in the torque band. But I'm not a high performance boater, what do I know.

Looks like those engines should revlimit like a car... Based in internetting. But it's an older one. Efi makes engine conrtrol so much easier. Regardless rev limit looks like it would be more towards 6k or so.