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Old 31 May 2010, 15:55   #1
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Mercury 90 down on power

Hi, we have a very very good condition 1998 Mercury 90 on our rib, the engine hasn't had a hard life and only let me down today. We went out for an hour at about 33 knots and not a problem. We then went up the Beaulieu river at 5 knots and after a while the engine started to run a little rough (like it had dropped a cyl). Went back out to sea and gave it full chat and up 33 knots again, did this for about an our and then with out a warning dropped back to 18 knots at full throttle. Any views on where to look first?
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Old 31 May 2010, 16:13   #2
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Could be so many things!, Best to see if it is dropping a cylinder first and identify which one. Once you have got that find out if its lost a spark or is it a fuel problem.
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Old 01 June 2010, 05:06   #3
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Could be so many things!, Best to see if it is dropping a cylinder first and identify which one. Once you have got that find out if its lost a spark or is it a fuel problem.
I would say its electrical because I could smell lots of un-burnt fuel when I flushed the engine out. My outboard man that looks after our engine said it may be a plug, coil or CD unit that has gone down.
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Old 01 June 2010, 06:21   #4
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Not sure what flavour yours is, but also check the HT leads if they are independant to the coils.

My going in guess would be electrical too.
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Old 01 June 2010, 17:51   #5
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Not sure what flavour yours is, but also check the HT leads if they are independant to the coils.

My going in guess would be electrical too.
Its a 90 ELPT


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Old 03 June 2010, 18:39   #6
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Just to give you an update, I have put in new plugs and swapped the coils around and its still the midle cylinder that isn't fireing, compression is 140psi on all cylinders so it may be fuel related
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Old 04 June 2010, 05:05   #7
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sounds like you need to dismantle / clean the middle carb...... or check all electrical connetions between the box of electronics & the plug.
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Old 04 June 2010, 12:17   #8
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Have you got a spark on the middle cylinder?

If you haven't, after the tests you've already done you'll know it's black box related.
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Old 05 June 2010, 10:44   #9
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Just found something very strange, when you disconect the ruber pipe that goes from the middle cylinder to the fuel pump the middle cylinder starts to fire, the fuel pump does have a new diaphram in it.
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Old 05 June 2010, 18:24   #10
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Clean the carbs. You've probably got a stuck float needle.
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