performance issue

[tautog_33]

Hi everyone i took the seacraft 20 sf out today the sea conditions were 2-3 chop. I could not get the engine to turn more than 4000 rpm. I took it out last week in more or less the same sea conditions and was cruising 4200 no problem with a WOT of 5200. I cant figure out why such a decrease in performance. The only thing I've done different from last time is i added 20 gallons of fuel with 4 OZ of marine STA-BIL and one more person. Could the fuel filter be clogged? Thanks in advance for any advice! Mercury 135 HP 1988.