Finally thought I had found the problem on my 525 poor performance. Have no codes, good fuel pressure, low hour motor, 100% available power. Been thinking sensors issue but after endless checking decided to go back to basic old school. Check with vacum gauge and had evidence of bad valve. Had great compression but decided to do leak down test. Getting to #8 it failed and could hear air coming threw intake. Ended up backing off the intake valve lash. My concern is when I took adjustment to zero lash and turned 3/4 in, the air would start coming out of intake, and then build back up and leak down again, like a pop off valve. So I left the lash at zero and it worked with no air bleeding out. But the boat doesn't perform much better, maybe a little. Still has several second delay at 2300 rpm and then takes off, and wot rpm is down 400 rpm. I also noticed the rocker nut appears to be up a lot more then others. Could I have a lifter stuck up? Slightly bent valve? I'm thinking that maybe was oil pressure is up maybe I'm loosing pressure on more than one cylinder? It never has backfired. Any thoughts?

Wow, sounds just like my starboard motor. No codes, passed all tests, just down 400rpm and a bunch of power. Been 2 years now. Start of last season I had the long block rebuilt and it did not fix it. Do you have a scanner or rinda software? Check your timing. My timing does not advance and I believe that is the cause (but what causes that is the question).

http://www.offshoreonly.com/forums/d...wners-fyi.html
This thread saved my a$$, Don't think you have the exact symptoms but sounds like a possibility.....
I used Marren Injection to service mine, worked perfect!

Yes did check with rinda software. Interesting mine looks like it has a mercury rebuilt tag on the block. Mine appears to advance up 27 degrees. As far as the injectors, I can test them with the rinda, but when I got to the other half the software says I didn't have the hardware to check? Still looking. Maybe someone has had similar problem and found the fix?