Yamaha F115 low RPM's at WOT

[rocket350]

2005 f115 on 19' Cobia CC. Engine has less than 200 hrs. I bought this boat a few months ago. 1st time I had it out it would reach 6000 rpms easily and cruise about 33 mph with the black ss 17" prop. I borrowed a 19" aluminum and got about 35mph at 5800 rpm but seems to feel worse after a few trips as the power, speed and rpms seemed to drop. I put my 17" back on yestrday and what a noticeable diference. I can get 5500 rpms by trimming the engine and maybe 29mph. It feels like there is less power. It isn't the same as what it was a month ago. Sometimes it will stall putting it in gear. Otherwise it runs great, starts/idles fine. No hesitation.

 

[99yam40]

You have noticed you motor is sick, take it to the motor doctor.
Hope you have a Good Yamaha shop close by with trained mechanic employed there so they can hook it up to a computer

newer marine motors are just like the newer automotive units, they have computers and need to be looked at by trained techs with the proper equipment

You can start trowing parts at it to see if that will fix it, but that can get expensive quickly.
If you would like to do it your self then a Leak down test on all cylinders is where I would start to make sure all are sealing properly, then test spark on all cylinders and timing.
Maybe test the fuel system pressures or replace/change the filters and screens. Injected motors have many
If all checks out OK with all those then get a computer and the proper software and connector and have a look.