‘05 15HP Johnson Outboard Issue

[EastTNHunter]

Took my jon boat out yesterday to hunt. On the return trip everything ran just fine for the first couple of miles, but then it started to "bog down" slowly despite the fact that I had the throttle held wide open. I let off the throttle to idle speed for just a few seconds, then opened it back up and she ran fine again for another 1/4 mile or so only to repeat itself over and over until we got back to the boat ramp. Ran fine at idle, no real "hiccups" or misses, and the "bogging down" was as smooth as if I had just let off the tiller throttle slowly. I'm using an inline fuel filter, and I see no major issues in it, so I really don't know where to start. Thoughts? I'm thinking ignition coil, but I don't know how to test it.

 

[TNCharlie]

I have no special knowledge about such things, just some experiences, so for what it is worth, here are a couple of thoughts.

My guess would be a fuel supply issue - losing pressure in the line to the tank maybe.

Maybe a partially blocked fuel tank cap vent.

Maybe a faulty fuel pump - I've seen similar symptoms where the culprit was the fuel pump.

And then there is the carburetor, which might just need a good cleaning but possibly replaced.

Again, these ideas are worth exactly what you paid for them and may have no validity at all.

 

[BlackEagle1]

I had that happen last year on an 2008 mercury. I finally figured out that the lining in the fuel line had separated from the hose itself. When at idle enough fuel could pass to feed the engine and keep it fed until the remainder of the fuel beyond the blockage was burned. At this point when the motor attempted to draw more fuel, the liner would collapse, and choke the motor out. It was very repeatable. New line and no issues. This was one of the silver ethanol safe fuel lines.

 

[jdb901]

Definitely a fuel issue I had the same thing happen before squeeze the primer bulb several times when it does this and it should stop the issue. Maybe open the vent cap on the gas can a little more and prime the line more at start up.

 

[Tenntrapper]

I had a similar issue with an old 9.5 hp. If it only runs WOT for a few seconds, I would suspect the fuel system....maybe a stuck float not allowing the bowl to fill up. My carb float had a plastic pivot pin, ethanol had caused it to swell and stick...lesson learned.
Another time the same motor would run great for maybe 1/4 mile or so, and then start loosing power. Letting it idle a minute would allow me to continue onward. That time it was a stuck thermostat...it was getting hot. New thermostat, and it was good to go.

 

[Big Ben]

Check the vent cap on the gas tank.