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Old 29 June 2009, 03:02   #1
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Any advice on where to start?

Hi all,

I was contacted yesterday by the new owner of my old dory (Mercury 50 HP 4 stroke on Carbs).

He wrote....."It is only running on 3 cylinders under load. I checked the carbs and cleaned them out which seemed to work for a couple of minutes then it began to run on 3 again. Also the water coming out of the telltale is cool until you turn the engine off then it continues for about a minute and the water becomes boiling hot. Is this normal? Also there is a high pitched alarm keeps sounding, Is this oil warning or temp?"

Has anyone any ideas where to start with this.....my initial thoughts were that it was an overheat problem possibly due to a duff thermostat.

I did a brief search through old posts and have cobbled together (dangerous I know) a 'theory'..........

Would i be correct in assuming that its got a temperature probe located somewhere in the head which will cut out the coil packs to shut down the engine if it gets too hot? "The probe switches to ground when the temp gets too high, sounding the alarm in the controls and putting the engine in 'get you home mode', revs are limited to about 2200-2500rpm"

I don't know is it definitely running on three or could it be in 'get you home' mode.

So in summary, my question is: Could it be a sticking /failing thermostat? If it is running on three is it possible it is shutting down individual coil packs and may be shutting down the one for the cylinder which is getting hottest first?

Any advice /thoughts greatly appreciated. I could (and probably am) miles off with the above, but it is on my conscience that the engine is playing up already....it was demonstrated running when sold, but don't like the idea of the new owner having problems.

Many thanks in advance,

Dan
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Old 29 June 2009, 03:46   #2
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Country: UK - Wales
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Dan

It could be a number of things. Thermostat unlikely as I think it would fail open and then the engine wouldn't get warm. It doesn't sound like an overheat problem to me.

How does he know its running on 3? and why only under load?

If he's cleaned the carbs out and kept the same fuel, could de dirt in there again. When he says he cleaned the carbs did he strip and rebuild and clean the jets?

When did it last have a good service?

I would start with-

New Fuel
Carb Strip and Clean
Spark Plugs & HT Leads
Oil & Filters
Impeller


Jono
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Old 29 June 2009, 03:47   #3
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Does sound like either the thermostat or impeller. Was it used predominantly in salt water?

The cooling passages can get salt build-up around the thermostat. With the thermostat removed, I'd clean the recess, flush with fresh water and install a new thermostat. Also, was the impeller new?
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Old 29 June 2009, 04:34   #4
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Cheers guys......

Jono - was serviced mid 2007 did max 10 hours that year and was overwinterised end 2007 and not used last year at all (was started twice on muffs to check). Seems odd that it worked okay post carb clean then back to 3 again........I also asked whether he had changed the fuel as the tank had been sitting since end of 2007!

Spartacus - yes was run in saltwater predominantly, think removal /replacement of thermostat and flush is in order. I believe the impellor was done 2006 when i got the motor. I had an overhaul /service when i bought it as I wanted to be sure all was right from day one......for belt and braces, don't think an impellor change would go amiss either.

Any other insight /advice greatly appreciated. Am I corrrect in my recolation that this is basically a Yamaha unit (is a circa 1999 model 50 HP 4 stroke carbs)?

Dan
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Old 29 June 2009, 04:42   #5
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Quote:
Originally Posted by Dan Worth View Post
Am I corrrect in my recolation that this is basically a Yamaha unit (is a circa 1999 model 50 HP 4 stroke carbs)?

May be a Yam block but thats all.

Not convinced about the thermostat, if your unsure take it out and test it in saucepan (or microwave)

Jono
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Old 29 June 2009, 04:50   #6
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could be a spark plug fouling up on a cylinder thats not getting hot enough or up to running temp , some engines its the lowest cylinder or the one that gets the cooling water first , spark plugs can sometimes show a strong spark when testing but back in the engine when under compression the spark can fade or get weak .
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Old 29 June 2009, 05:05   #7
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As far as the cooling is concerned that model is prone to a midlife crisis similar to prostrate problems! There is apparently a rubber water seal at the base of the powerhead similar to a very small doughnut. When the doughnut becomes a bagel you have problems. The bit costs about 5-getting to it to replace it costs considerably more. Our 50merc fourstroke that we sold 3 years ago is apparently now undergoing surgery for it despite having been cosseted all its life both by us and its current owner.
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Old 29 June 2009, 08:54   #8
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Thanks Dave,

I spoke to an outboard mech and he offerred the same diagnosis (without seeing the engine at this stage).....quoted circa 300 for the work (ballpark figure).

I have spoken with the new owner and we have come to an agreement re getting the work done........wouldn't sit well with me otherwise and without getting all buddhist think it would be bad karma not too!

Thanks for all your replies, will let you know the diagnosis etc when the chap has looked at the engine...........bl**dy boats!

Dan

PS Dave, did i do my advanced PB commercial exam with you on the Menai Straits?.....you may remember that I was the one who pointed up when I meant down and vice versa (in relation to trim angle)........It was a very cold November day /night on that boat!
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Old 29 June 2009, 09:21   #9
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yeah thats it I remember the name now. trim up/out down/in - all too much at night with a grumpy old git putting ya under pressure at 30 knots with horizontal hail eh! trust you have recovered from the experience and no longer need the valium!
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Old 02 July 2009, 08:08   #10
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All recovered thanks Dave.........still need the valium though as we have had a happy and very active little 4 month old son since i saw you last!
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