15HP 2 stroke bogging down @ WOT

[rneurohr1]

I have the above on a 14' Fisher aluminum V hull. Took it out this morning, for the first 15-20 minutes the engine was running great, really had the boat moving and on plane. After about 20 minutes, I noticed that the RPM @ WOT was dropping and the boat was acting like it was bogging down, guestimate of running at half the speed it had been earlier @ WOT. I cut the throttle to idle speed for about 10 seconds then went WOT, boat again ran great, fast, and on plane for about 2 minutes then bogged again. I have a newly rebuilt carb and fuel pump, fresh fuel, cleaned the plugs last week. Looking for suggestions on where to start with fixing this. Thanks in advance.

 

[rneurohr1]

Re: 15HP 2 stroke bogging down @ WOT

Forgot to mention it is a Mariner, model year 1989, #OC177406, also have a new fuel line and primer bulb.

 

[NumberGuy]

Re: 15HP 2 stroke bogging down @ WOT

Hello, and welcome to the forum.

You mentioned you had recently rebuilt your carburetor, which immediately perked my ears. It sounds to me like your needle valve inlet is sticking, which can account for your bowl filling with gas at low throttle, having enough to get you going for an enjoyable ride, and then is depleting after a few minutes at high speed. It does not seem like your fuel pump is the issue because you are able to run at high speed for a significant amount of time before it dies, which shows that the pump is indeed pushing a sufficient amount of fuel to the system. My question for you is how far did you go with the rebuild?

 

[rneurohr1]

Re: 15HP 2 stroke bogging down @ WOT

I had the local Mercury dealer do the rebuild, they used a rebuild kit which I assume would be a complete and total rebuild. After thinking about this, I just got the motor back from the dealer in early October and until it ran great earlier this week, it was bogging down every time I took it out, I just did not know it as I had not seend it run any other way. I spoek with a friend who has been around boats for years and he suggested that it may be a coil as running it in colder temps may allow it to work but once it heats up, something expands and it does not work properly, thus the motor runs on 1 cylinder. Does this make sense? As far as I know, they are the oiriginal coils so being 22 years old, maybe it is time to replace them.