Tohatsu M40D2 3000 RPM Max

Hello, firstly this is my first post here and I am an aspiring boat mechanic so I don't know an awful lot so explaining more in depth would help me out a lot. I have a 1999 40HP M40D2 that would run great 2 months ago despite some starting issues. Since it had trouble starting I recently just had all the carburetors cleaned and rebuilt by a mechanic and had no problems. Fast forward a week and now the boat won't go above 3000 RPM when in gear, if I crank up the throttle it'll reach normal RPM. I changed the fuel line and all 3 spark plugs by myself and the boat still won't go max speed when in gear. Any help would be appreciated and explaining it a little more in depth for me would be awesome, thank y'all!

Check for a spun prop. Verify fuel delivery. Verify timing.