Boat Won’t Go Over 3000 RPM Under Load

Hi everybody, my name is Witold and hello from New Zealand
My Mercury two stroke 1988 year serial no.0B373318 80hp cant go more than 3000rp over load.

Last time when i was fishing boat suddenly slow to 3000rpm, when i came back and open engine cover i notice that plastic clip what hold two control wire was open...so i put it back and close, meanwhile was knockdown in New Zealand, so i changed water pump impeller because i had plenty of time...
Today i was on water and noticed that i cant go over 3000rpm... when i lifted engine above water i was able to do max rpm... I was sure that it was because of open plastic clip what holds control wires and accelerator wire cant be pulled to 100%... but it looks that issue is still
Engine starts and goes as usually...

I read some tips in google and i going to change fuel filter and clean carburetor, any advises whats different can be issue?
I read about safety neutral switch that can be source of problem as well? I have got quicksilver control...

I spend 5 years in car mechanic school 20 years ago so maybe I will be able to check it yourself...

Thank you for help, and greetings from New Zealand!

Step #1 is a compression test , post your numbers.-------Then test for spark.---Good spark can jump a gap of 3/8" ( 1 cm ) with ease.----Neutral switch prevents starter from operating if motor is in gear !

You may have lost a cylinder due to bad plug, coil, water getting in, etc. Or you may have a carby problem. you need to test and find out which/ what/ where.

Jeff