90 hp mariner loading up

I have a 90 hp mariner on my pontoon boat and it seems to run ok first thing in the morning and with only a few people on it but after loading it with more people or trying to tow a tube for my kids it seems to load up and not have any power. By the end of the day I am lucky to get back to the dock. But the next morning it will run ok and then do the same thing again. Any suggestions on what it could be? I have had a mechanic work on it but he just charges a lot and it never seems to run any better. He has changed the jets which I am still trying to figure out if they are the right size and where he is getting his information. I live at 7500 ft and I have changed the prop to the recommended pitch to get more rpms but it only seemed to make it slower and not fix the problem. I don't know what to do and I am lost any help would be greatly appreciated.

Re: 90 hp mariner loading up

Troubleshooting any problem is best done with a few simple tests.

Begin with a simple spark plug inspection, first take a short warm up ride, when the motor is running fine shut it down, pull the spark plugs and inspect each, they should have a uniform appearance and be slightly wetted with oil. Reinstall and enjoy your day. When the motor is loosing power, pull the spark plugs again and reinspect, describe their condition now.

With the motor OFF and the trim adjusted level, remove the airbox cover and pump the primer bulb, hold a firm but not bone crushing pressure on the bulb while looking down the throat of each carb. Do you see ANY fuel leaking into any of the carbs? It may help to shift into Forward gear and the throttle wide open so that you can see farther into each carb.

Compression test each cyl.

Spark test each plug wire.

Even one flooding carb will overfuel a cyl, the recirc system will spread the excess fuel throughout the motor and can lead to fouling spark plugs.

Test the overheat alarm system to ensure it is functioning. With the key ON, disconnect the tan/blue wire connected to the temp sensor on the cyl head area of the motor, ground the wire, the temp alarm should sound. You need to be sure it works, an overheating motor will loose power and can cause extensive damage if continued to run.

Check these things and repost your findings.