Well, I can't be the only guy out there that has a '90 15Hp Mariner outboard motor.

When looking at the back of the motor, up under the engine are there two exhaust ports or just one?

I suspect there are supposed to be two and my motor has a manufacturing defect in that the second hole wasn't drilled all the way through.

 

Can't answer your question, but can refer you to iboats.com.

I've found quite a bit of help over there.

Ed

 

It really shouldn't matter, they are just relief ports for when the engine is idling. When under power there is enough exhaust pressure to push it out through the propeller hub.

 

Thanks for the link,
I know it shouldn't make too much of a difference at high speed but at slow speed it is kind of the difference between breathing through one straw or two... Apperantly the motor was having a carbon issue. so we'll have to see if drilling it out helps....

 

Are you using good oil?

 

So far, I'm not using any oil.... The motor is new to me. But I generally run good oil with a slightly higher grade gasoline.

 

westcoaster, by any chance do your exhaust ports point down?

 

Yes, they are pointed at the prop and are part of the leg. it's a bump on the housing up at the top.

 

Ok, because last night a saw a 15 HP Mariner last night on a boat and its ports were pointed down. It had the two ports but like yours only used one of them. On;t one had the black streak where exhaust gas came out.