Yamaha 150 runs poorly when hot

[octaneforce]

2004 yamaha 150 v6 carb'ed. Engine was running very poorly with no bottom end power. Normally my boat leaps out of the water and it was struggling to get on plane. I changed all the plugs (the old ones looked black and carbonized) and the boat ran perfect for about 25 minutes. After that it went right back to the way it was. I dont know where to start.

 

[octaneforce]

Also replaced the water separator when i did the plugs

 

[99yam40]

Start with the basics
compression, spark and timing
then fuel pressures
test what you can to make sure things are OK the when it acts up test again to see if something changed

 

[robert graham]

Is motor temperature coming up to ideal running temperature?.....Is the fuel old or contaminated?....