1996 yamaha 115 knocking

[nasdaqsam]

After 19 years of a solid performer the ole girl has finally developed an issue that might be the end of her. The engine will run up to about 3000 rpm's, will not make power and has a knock I can not determine where it is coming from. We did a compression test three are running right at the 110 the lower right is about 105. He used a scope the cylinder walls are all pristine with no damage at all at least from what can be seen. I had a semi boat mechanic look at it, short of tearing it apart he is thinking wrist pin knock.

I am looking to eliminate as many of the things that can be checked short of opening her up as possible before she has to come apart. Thoughts?

If we do have to get into the motor I have been hearing horror stories about changing out the pistons/rings rods etc. The parts themselves are fairly reasonable. I have a very good car mechanic that is willing to tear it apart but he has never worked on an outboard. Are they that difficult to take apart replace the pistons and put it back together?

This is going to have to be a do it yourself fix as the old boat isn't worth a major rebuild and even if it was the only few boat shops around here are booked way out.

 

[robert graham]

I would want to find out if the 3000 RPM maximum and the knock are related/connected to the same issue....sometimes a drop in RPM's is loss of power in one cylinder....maybe a dirty/plugged carb causing a lean fuel mixture/noise?....just a thought...

 

[nasdaqsam]

Robert, We checked the fuel system that was a mess. The fuel line was completely deteriorated and the bulb had a split in it. We replaced those and cleaned the filter and carbs out they were pretty gummed up with rubber from the fuel line. That did not change the rpm thing or the knock. It could be this created a lien condition that effected it as you say.

From what I can hear it all sounds like it is coming from the lower right cylinder. We are going to have to split it to find out for sure it looks like. One thing to note if it does turn out to be that cylinder is a few years back maybe 5 I changed the spark plugs before going on a big long run about an hour and half away one way. The boat seemed to run fine all the way it was a nice slow ride so never pushing the rpm's. When we got there I noticed we used way more fuel than we should have thats when I found one of the plug wires had come unhooked. I put it back on and she has never missed a beat since. I just wonder if some damaged was not done way back then.

 

[robert graham]

When he scoped it did he look for signs of overheated/melted piston crowns that might have been caused by a lean fuel mixture/plugged carb jet?.....

 

[nasdaqsam]

He did and didn't see anything. The piston tops and cylinder walls were like new.

Since I can not get anyone to look at the boat for quite some time much less do a rebuild. I have decided to keep trying to figure this out until it comes to what ever conclusion. The boat is in better than new condition just not worth a new motor. In the mean time I just picked up a 2007 South Bay Trifecta with a 496 HO with just 42 hours on her. The problems with finding a boat mechanic in high season when I need the boat was the deciding factor. We do not get a lot of days on the lake here waiting a month for a fix is not an option. I have a half dozen top notch Chevy mechanics that could work on it anytime I need.

I still want to get this back up and running to either take to out Florida place or sell though. She's a great boat.