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Old 19 July 2006, 10:23   #1
42k
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Country: USA
Town: Columbia, SC
Boat name: U S Sea Cockaboat
Make: Scout
Length: 6m +
Engine: Yamaha 100 four stro
Join Date: Jul 2006
Posts: 5
Power lost

Just found this board and it looks like a very knowlegable crowd here, so here goes. I recently had a problem with my 100 horse power 4 stroke Yamaha engine. I had 4 carb kits done, replaced plugs, pumped fuel from tank (dirty or water in fuel) and replaced filters. Boat ran great for about a month. Then it started to cut back on power after about a minute of running. Almost as if the fuel wasn't pumping enough. It would do this every time after I would stop and start again. I doesn't run rough but runs at about 75% power. What do you guys think?
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Old 19 July 2006, 11:03   #2
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Country: UK - Wales
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Boat name: Too Blue
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Could be fuel starvation. Either a blocked tank breather or a kinked pipe where it goes into the engine. Had a similar prob with my outboard - just pulled the fuel pipe out a bit and problem solved.

To check for a blocked breather try opening the fuel cap - if there is a hiss of air or the engine suddenly runs fine then that's the problem.

After re - reading your problem though it could still be much in the tank - check your filters again - they may be blocked.
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Old 19 July 2006, 13:48   #3
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Country: USA
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Boat name: U S Sea Cockaboat
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Length: 6m +
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Join Date: Jul 2006
Posts: 5
Thanks codprawn, that's what i will try. I replaced the filter. By breather do you mean the pump bulb?
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Old 19 July 2006, 14:04   #4
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Country: UK - Scotland
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Quote:
Originally Posted by 42k
Thanks codprawn, that's what i will try. I replaced the filter. By breather do you mean the pump bulb?
Whilst my name isn't Codprawn ( ), I think he means the fuel tank vent.
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Old 19 July 2006, 14:18   #5
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Country: USA
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Boat name: U S Sea Cockaboat
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Length: 6m +
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Posts: 5
Thanks jwalker. Sometimes you UK guys have different terminology!
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Old 19 July 2006, 14:38   #6
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Country: Other
Town: Oakley
Boat name: Zerstörer
Make: Ribcraft
Length: 5m +
Engine: Suzuki DF 140
MMSI: 235050131
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I had a similar problem on my Yamaha last year. Water had got in to the control wires and was causing the electric choke to short and to come on.
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Old 19 July 2006, 14:52   #7
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Country: USA
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Boat name: U S Sea Cockaboat
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Length: 6m +
Engine: Yamaha 100 four stro
Join Date: Jul 2006
Posts: 5
Biggles, What did you do to fix it and where is that located? Again it runs fine for about thiry seconds and then loses about 25% power but runs smooth.
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Old 19 July 2006, 15:15   #8
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42K. As the engine was due a service I got them to sort it out. It needed the carbs cleaning which I thought was the problem but when I went to pick it up they told me about the choke.

I suppose to check it you would need to run it with the cowl off and see if the choke came on even if the engine was warm.

The symptoms for me would be that the engine would generally run fine up to 4000 revs then if I went any higher it quite quickly started to bog down and run at about 2500 revs but not erratically. If I rested the engine it would then be okay until I went fast again.

My control box had had all the cables cut and rejoined when I bought the setup. It was at the join I got problems with corrosion. Since they did a good job of it I have not had anymore problems and she runs as sweet as a nut.

Strange thing is I nce had a car where the choke stuck on and that didn't really seem to affect the performance anywhere near as much. But it was easier to fix.

Best of luck 42K and if you don't get much luck here why not try http://www.iboats.com/bbboard/html/mainpage.html now they really know there engines.
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Old 19 July 2006, 15:30   #9
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Country: USA
Town: Columbia, SC
Boat name: U S Sea Cockaboat
Make: Scout
Length: 6m +
Engine: Yamaha 100 four stro
Join Date: Jul 2006
Posts: 5
Thanks Biggles, I'll give the site a look.
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Old 19 July 2006, 17:45   #10
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When I had the problem my fuel computer was a great help - I KNEW something was very wrong as when I opened the throttle the fuel consumption got better - 6gph at full throttle - I wish!!!
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