RPM's on Starboard Engine Declining

[TOGA]

So last week I started noting soot build up on stern after a lot of idling around marinas.

I ran nearly a tank of fuel through it this weekend loaded with SeaFoam. Didn't do a lot of idling, but there wasn't a noticeable amount of soot this week. But then again, I've never seen the soot build up before the weekend of touring at low speed, so not sure that indicates anything.

Some suggested changing the plugs which I am going to do this week.

Next question without trying to overthink in advance of changing the plugs...

This week the starboard engine would not go over 9500 RPM's. Both engines have always consistently performed at 10,500. The port engine was the one producing the most soot.

Top speed this week was 43. Normally i can get 47-48. Peak three years ago was 52.

Anything else I need to be looking at?

This may be our last season with this boat as the family is outgrowing the water sports and wants a cruiser they can spend the weekend on.

We have a long season here in Georgia and we normally use the boat 3 weekends a month well into October / November. I want to make sure the boat is in top working order for the rest of the season as well as for a potential new owner. It's been a great boat and I want to make sure I pass it along in top performing order.

Thanks for any thoughts and guidance.

 

[Scottintexas]

spark plugs, I once had an engine that would only go 8k, changed the plugs and it was 10k again,
I would suggest a YDS unit just to help understand your engines but if your going to sell it I wouldn't bother,

Plugs first, I wouldn't start to worry or overthink it until after that step is done,

 

[LOZBum]

I agree totally with spark plugs. Just yesterday mine was idling rough and wouldn't get peak rpms. I had a set of new plugs that I bought last year and forgot to put in on the boat. Went back to dock, spent 30 minutes putting plugs in and all issues went away.