2005 Mercury 9.9 pro kicker No power under load

I have a 2005 9.9 pro kicker...I have the strangest problem. Motor starts fine, idles fine powers up fine like 5500 RPM while on muffs even in forward gear..again on muffs. Will Ideal forever and fun forever even at high RPMS on muffs...took it to the lake and started fine..idled fine...put at full throttle won't go over 1600 RPMS...like no power under load...Again..will run forever..just no power..never sputters..just no power...I have clean the crab..put on all new fuel lines...even a new fuel line from the tank to the motor...I have check the exhaust..no clogs...I have checked the plugs..both clean and firing even...I am totally lost...I have tons of RPMS and Top end power on the trailer out of water..using just muffs..but once in water...NOTHING..won't push my 14 foot v bottom boat over 6 MPH..my 45lb thrust electric motor pushes it 1.5 mph. Also RPM speed in gear should be 900 RPMS...I had that set on trailer..actually was like 920 (figured water would slow it a bit) when I put it in gear on lake..dropped to 520...I took motor cover off..to get it to idle at 900 RPMS in gear...I had the idle screw moved in so far..I actually couldn't shift it back to natural. I shut the motor off..turned it back on in neutral and ideal RPM was like 1400 RPMS..way to high to shift in and out of....Crazy..anybody have any ideas why I have tons of power on the trailer..but once in water..very little?????? Never had the problem till now. Thanks.

 

Well, for starters idle speed is always a few hundred rpm lower in the water, because of the back-pressure of being underwater, rather than just in the air. Hard to see how you could have lost spark advance, but that would have the same effect. Have you done a compression check, if it was way down on one cylinder that would make it lose power dramatically. Or is it only running on one cylinder, due to no spark (faulty coil, e.g.), ditto . Check the plugs to see if both look the same. But that would make the engine hard to start, presumably. However, the old story applies, you are better to get a workshop manual and read up on troubleshooting.