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Old 09-01-2017, 07:57 PM   #1
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Dinghy Outboard Won't Run High Speed

Trying to track down a problem with my Merc 4HP 4-stroke dinghy outboard.

It starts fine and runs great at moderate speed. However, when I open up the throttle, it sputters and won't reach a smooth top speed.

I suspect a fuel issue. I've changed out the fuel filter and put in a new spark plug. The old one looked exactly like the "perfect" example on the spark plug charts I checked. Of course I've run hours' worth of fresh gas through it. I even bought a gallon of that $20 engineered fuel they sell in the home supply stores and ran a tank of that through.

Next steps? Carburetor rebuild? Something else in the fuel system?

Hoping someone has seen this before and can point me down the right path.
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Old 09-01-2017, 08:26 PM   #2
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Just clean the carb. Probably junk in high speed jet. 20 minute job
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Old 09-01-2017, 08:52 PM   #3
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Just clean the carb. Something is in the high speed jet. Usually the low speed jet gets plugged first.
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Old 09-01-2017, 10:39 PM   #4
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THird the high speed jet
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Old 09-02-2017, 01:45 AM   #5
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Air leaks into the fuel hose can do that too.
Fwiw on a Yamaha, the fuel to the low speed jet goes through the high speed jet first. So if the high speed jet is clogged, low speed operations are degraded as well.
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Old 09-02-2017, 06:29 AM   #6
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Thanks! I was thinking (hoping) it was the high-speed jet, caused by a really bad batch of fuel I was forced to use in a remote port last year. It got a little better with fresh fuel, but then seemed to plateau. Even running hard for long periods hasn't improved it much.

I'll clean the carb this winter, but since I can't run wide-open in a barrel, I won't know if it's fixed until spring, so I wanted to know if there were other places I should be looking. Air leak in the fuel line is something I'll check, too.
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Old 09-02-2017, 07:39 AM   #7
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While I like others suspect the carburetor, there is one other thing to check: the fuel filter and if you have an on engine fuel tank, the bottom fittings.

Years, and years ago I had a dirt bike that did the same thing. Removed the tank and found a plug of dirt in the fuel fitting from the bottom. Cleaned it up, changed the filter and it ran fine.

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Old 09-02-2017, 08:29 AM   #8
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One of the guys at my marina had what sounds like a similar problem and a new fuel hose/squeeze ball fixed it.
The old stuff had delaminated.
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