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Old 28 January 2010, 17:09   #1
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Country: UK - England
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Engine: Yamaha 50hp 4stroke
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1995 Yamaha 50 hp Fourstroke Engine Problems

1995 ... Yamaha 50hp outboard engine ... model code F50 .. Four Stroke Engine ... Carb Model ... Any ideas anyone

Have on a couple of occasions been motoring alone around the 16 - 22 knots speed and the engine has been sweet, no misfiring, no abnormal emissions (Smoking etc)

Out the blue, the engine rev's are restricted as if the safety device has kicked in, though there is no audible buzzer warning. There doesn’t appear to be any overheating issue going on. With the engine cowling removed, the fuel filter is full and no sign of debris in the fuel filter housing.

With regards me being inclined to think its not overheating, I can actually place my hand on the cylinder head at various positions and the temperature is uniform and it appears to be no hotter on occasions when it’s been running trouble free (not scientific I know).

Water is pumping fine at various water outlets and when placing my hand in the flow of water it doesn’t appear to be any hotter then it has been over the many years we have owned it.

We cannot replicate this condition when running the engine with muffs … it simply runs fine, full rev range available.

The fuel-priming ball is as it is normally, and all cylinders appear to be effected, there is no misfiring on a particular single cylinder.

The engine can run fine for a couple of miles/kilometres at any speed we decide to run when the onset of the fault is sudden. We cannot anticipate when it is likely to happen.

Has anyone heard of one of these engine going into a “safe mode” when there is no apparent reason that it should. Are there any diagnostic to check out this system? … or is it likely to be some other cause you, might have heard about with these engines, and whats the fix?

It has us baffled .. any helpful advise would be much appreciated.
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Old 28 January 2010, 17:22   #2
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Is the oil level ok?
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Old 28 January 2010, 18:59   #3
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Hiya ….
Yep the oil level is within the upper/lower dipstick markers, also the oil pressure warning light illuminates when the ignition is switched on, started up, it’s off instantaneously.
I was wondering if it might be the CDi unit playing up intermittently, the only thing that is putting doubt into my mind regarding the CDi unit is that it takes a couple of miles running before the problem happens.

I’ve got the Clymer manual and intend on running through the fault finding sections, the problem has only ever happened when afloat. We have tried to replica the problem running the engine with the muffs but no luck.

Impellor replaced a couple of season ago and its pumping the water out as normal. No obvious signs of engine over heating, as in the head/block doesn’t feel hotter than it has normally been over the years we have owned it.

I was just curious to know if someone in the past has had spurious signals from the CDi unit in the past that has made the engine run in “Safe Mode” … Also is it possible that the CDi unit has the capacity to do this?
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Old 29 January 2010, 04:10   #4
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Not a Yam , but I had a similar problem wit han old Suz DT25. It started occasionally going in the huff for no apparent reason, and like yours, no alarms etc, it would just go in the huff.

Eventually it started becoming so repeatable that I could start it no problem, and it would take off no problem, but would exactly 20 seconds after opening throttle hesitate, and a further 20 seconds later, die to idle. Absolutely repeatably. Would happily idle all day.

Turned out to be the throttle sensor had packed up, and was giving a constant reading of "closed throttle". Thing is, the CDI was presumably not programmed with "sensor fault" alarms, and only got confused when it got high RPM reading with "no throttle".

The Clymer for mine had a full set of sensor tests, assuming yours does, might be wiorth checking them all out. - you may have a sensor being a bit intermittent, so worth checking all connectors as well.
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Old 29 January 2010, 09:25   #5
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Cheers m8 ... out of interest is your engine fuel injected?
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Old 29 January 2010, 10:30   #6
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have you tryed some new spark plugs ,,i had problems with a near new engine ,, turned out it was a plug that had a crack in the ceramic electrode ,couldent see it at first on close inspection , and it always showed a spark ,but only missfired when under load ,new set of plugs and it cured it ,
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Old 29 January 2010, 11:22   #7
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Have tried new spark plugs, more so as a service item ... the engine doesn't misfire, it simply has reduced rev's ... it runs fine on it's limited rev range, but the problem is it wont go over approx 2400rpm

Someone mentioned oil level ... it’s a third of the way up the crosshatched marking ... so it’s within what Yamaha stipulate.

I'm going to give it a good looking at with my multimeter with reference to my Clymer manual ... all I can think of is to check out the sensors, etc, if they prove ok, it pointing to the CDi unit (not a cheap item) and its still not going to be a 100% for sure.

With it being a 1995 carb model it doesn't appear to have a processor that logs any faults that can be read via a diagnostic tester, or so I don’t think it has.
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Old 29 January 2010, 11:45   #8
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Pull the plugs - if one cyl is particularly wet, check the lead. My Merc ran like a dog when I got it, I then discovered that one of the HT leads was broken inside, and making intermittant contact!

If it;s holding 2400, that might hint at a "safe" mode, buit not knowing Honda's thought process on engine protection, can only second guess. 2400 was about all I could get on the suz with one cyl down. problem is if it;s intermittent, you could chase iot for ages & still not find it.

Could try if it;s running OK, pull a plug, & see how fast it goes.
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Old 29 January 2010, 15:38   #9
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Hoggy hello there . OK Im going too start off here with the basic's .I know you are probably going too think what's this GUY on about as I have checked all this already . HAVE you done a compression test with the engine warm ( what are the RESULT'S ) , have you done a spark test when the engine is warm ?? ( good spark , weak spark ?? ) and have you done a fuel test and checked the carb's are properly syncronized to the manifold's with a set of Vacuum gauge's ?? Have you had the carb's off ?? if so have you checked the high spped jet's and venturi for any debris or dirt in there ?? . Have you had the fuel lift pump off and checked the fuel pump diaphram's ?? I work and do work for a big yamaha dealer on the south coast and I have seen a few f50's ( carb engine's ) with this type of problem all relating to the FUEL system . Ok first thing I would do is check the fuel pressure the boat is delivering to the lift pump . now the ONLY way you are going to do this is by putting a fuel pressure gauge inbetween the lift pump and primer bulb , second thing is check on the fuel pump itself , this is controlled by the camshaft lobe which pushes the spring loaded shaft up and down on every revolution . THESE shafts and spring's do tend to STICK and can cause allot of upset and fuel prob's . It's VERY difficuilt too try and diagnose and outboard with this type of issue if you havent checked these type of items with the special tools ( Vacuum gauges etc ) . Have you had the thermostat out . An engine like this running too cold and the thermostat not working will send a signal to the temperature switch on the cylinder head which will send a signal too the ECM saying " Im tooooooo cold shut me down . Check this MATE . . HOPE THIS help's . Cheers Barry
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Old 29 January 2010, 15:39   #10
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sorry also you cannot hook up the YDS ( Yamaha Diagnostic's System ) to this engine
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