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Old 14 April 2016, 03:57   #1
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Yamaha 40hp 2 Stroke Intermittent Fault.

Our 5 metre rib Yam 40 engine has a very annoying intermittent fault. Out of nowhere it goes from running sweetly to cutting out/not reving/not starting. It is well serviced by a professional outboard mech. He has confirmed there is no issue as far as he can tell. Typically, when he sees it it's working fine. All he can recommend is a new CDI but does say in 40 years of working he's only replaced a couple.

The boat is used as rescue cover for sailing dinghies so does spend a lot of time on tickover. But is really only used a max of 5 hours a week. It is in a freshwater lake.

Anyone else any other ideas? I've tried numerous lines and tanks as I have access to a few. Does sound like fuel but we have kind of eliminated that.

Or do we have to try for the CDI? At around 1,000 to replace want to make sure everything else has been ruled out.

Many thanks.
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Old 14 April 2016, 04:30   #2
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Does the engine die away very suddenly say in two/three seconds or fade away over ten/fifteen? how do you get it started again? does it start miss firing before it stops? does squeezing the fuel bulb as its dying away regain it back to life. Do you have to sit and wait or keep churning the key? Is there any regularity to it i.e. rough seas or after a long day of use or after long flat out sessions etc I think if the engine cuts out again I'd have a spark plug spanner handy and before I tried to start it again I'd have a good look at the plugs. wet/dry etc Then i'd turn her over looking for a spark on each cylinder. I know its not the most pleasant thing to be doing at sea but its prob a great way to get to the bottom of it. On dry land with a cup of tea i'd do a compression check because its easy and rules out 60% of your problems.! Keep us posted Nik
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Old 14 April 2016, 05:02   #3
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>>>I'd have a spark plug spanner handy and before I tried to start it again I'd have a good look at the plugs. wet/dry etc Then i'd turn her over looking for a spark on each cylinder. I know its not the most pleasant thing to be doing at sea but its prob a great way to get to the bottom of it.

Yep totally agree... back to the old fashioned basics of any engine with running issues... albeit as said above not quite as easy at sea but that's the crucial time to gain evidence.
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Old 14 April 2016, 07:11   #4
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Quote:
Originally Posted by MattB View Post
It is in a freshwater lake.
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Originally Posted by matata View Post
I know its not the most pleasant thing to be doing at sea but
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Originally Posted by Fenlander View Post
I know its not the most pleasant thing to be doing at sea but its prob a great way to get to the bottom of it.

....albeit as said above not quite as easy at sea but that's the crucial time to gain evidence.
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Old 14 April 2016, 07:21   #5
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Come on Willk... sea... on the water... whichever... still harder than in the workshop where you stand by the outboard and put things on a bench.

And salty or fresh a spark plug makes the same "plop" when you drop it in.
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Old 14 April 2016, 07:43   #6
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Come on Willk... sea... on the water... whichever... still harder than in the workshop where you stand by the outboard and put things on a bench.

And salty or fresh a spark plug makes the same "plop" when you drop it in.
No no, I insist I'm right!!

My definition of "at sea" does not include a scenario where you can pull a 5m boat stern to a pebbly lake shore and stand in your wellies while you change a spark plug. That's more like my garden at present!!
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Old 14 April 2016, 07:49   #7
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>>>RIBnet admin team

Well yes... "of course" you're right.
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Old 14 April 2016, 11:36   #8
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engine has a very annoying intermittent fault.
Quote:
It is well serviced by a professional outboard mech.
Quote:
He has confirmed there is no issue as far as he can tell.
To me those three things don't really add up.
Quote:
But is really only used a max of 5 hours a week.
I know you think it does not have a hard life but it must be > 10 yrs old (2 stroke), if its used 5h per week for 10yrs its also no spring chicken in outboard terms... I'd certainly not want to spend ?25%? of the new cost on a "maybe" fix for an engine that has something like a couple of thousand hours on it.
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Old 14 April 2016, 14:15   #9
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Is it an Origin (2cyl) or Hitec (3cyl)?
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Old 14 April 2016, 14:48   #10
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Spark plug leads breaking down?
Hole in fuel pump Diaphram?
High resistance connection (corroded) wire.
Non return valve failure on primer bulb.
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