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.

Just clean the carb. Probably junk in high speed jet. 20 minute job

Just clean the carb. Something is in the high speed jet. Usually the low speed jet gets plugged first.

THird the high speed jet

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.

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.

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.

David

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.