Loss of speed 1995 225

Hope some one has an idea or two. My boat a 1987 Sportcraft Walkaround, 20.5 ft. will only due 34-36 at 5200rpm. When I bought the boat the fellow said it would due 50-52. I've had the boat a year and only started useing it weekly. My first trip out about 6 months ago it did 45 at 5600 rpm. Over the past few weeks the speed would drop a mile or two. Now the last trip was 34-36. I'm using a GPS. The boat is trailered, but has spent a week or two in the water.The motor is a yamaha 225 1995 Seems to run fine. Have noticed the lower RPM's at WOT, also idles lower 700 in gear, used to be 1000.
Any Advice
Thanks John

[boats.net]

Of course there are a lot of factors in boat speed. Weather conditions, hull conditions, load in boat etc... If the engine is loosing rpm's it will drop mph but it is unusual for a 400 rpm drop to result in a 10 mph drop. You can check the compression and ignition system on motor, may need carbs and fuel pumps rebuilt. Check condition of prop.
That engine should idle at 700 rpm in neutral and 650 in gear.

Thanks The weather and hull conditions are the same. How could I tell if the boat absorbed water in it's foam. Would I notice the water line lower? Have to be a lot of water.
I'm going to take the boat out this weekend and see how it does.
Thanks John

[jb123]

you should try an auxillary tank, hose and ball....see if it IS the motor or supply side.

also, while running, choke a bit...if it helps its a carb issue.