1989 mercury 90 hp 2 stroke won't loses power at wot

I have a 1989 90hp mercury 3 cylinder 2 stroke. I have installed a new stator, new switch box, rebuilt the fuel pump, new oil and fuel lines, tried an external fuel tank and hose, installed new reed valves and cleaned the carburetors. All the jets have been out of the carbs to make sure they're clear. The carbs were soaked overnight in carb cleaner. New spark plugs and new fuel also. It starts and runs great in the driveway. I get to the lake and it is really hard to start with the motor submerged in the lake. But it idles great once it's started. It just will not go full speed. It will go about 20 mph on my 18' aluminum lund boat. If I put the throttle all the way down, it boggs down to nothing. But if I keep it about 2/3 throttle, it will go about 20. I also did a test to make sure the trigger is good with a timing light. It checked out good. It's only firing on the respective cylinders when they're are suppose to. It has strong spark on all cylinders and has 120 psi compressionon all 3 cylinders also. I was told that maybe I have a clogged exhaust plate or exhaust passages? Any help would be much appreciated. Thanks in advance. Lastly all the parts I replaced have been in the last 3 weeks.

will the spark jump 7/16 inch with a gap tester?