2002 90 two stroke losing top end power

I have a 2002 90 TLRA that is only getting about 4400 rpm's. It is on a 20' Crestliner 2072 semi-v jon boat. It originally had a 17p Yamaha prop on it. The boat didn't originally have a tach on it but when shifted into gear it would almost sit you down and it ran really good. It noticeably lost power and sometimes the bulb would go flat at wot and the engine would cut out. If the boat was stopped and the bulb pumped, it might run several more trips before it would happen again. At some point the prop finally got so beat up I had to replace it so I installed a tach. The best I could do then was 4700 rpm's from a 15p. I was stationed in Nevada at the time a chalked it all up to the altitude. Well, I am retired now and back in Eastern NC so altitude is definitely not the problem. Now my rpm's are only 4400 with a 15p hustler aluminum prop and the bulb will still go flat at wot, but not every trip. I've cleaned the carbs, replaced the fuel pump, fuel filter, and bulb. None of this made any difference. Tonight I pulled the fuel line from the tank side of the bulb. No restrictions and looks good otherwise. I pulled the tube from the tank thinking maybe the screen was stopped up but it was clear as was the tube. The fitting on the tank had a check valve. When I pulled it I noticed that the threads were boogered up like it was crossthreaded from the factory. Otherwise it looked good and the checked valve seemed to function properly but I'll go into town tomorrow and try to find a replacement check-valve and fitting. I also did a compression check, 118 on all three cylinders, to rule out any internal problems. I guess the next thing is to take it to the shop and have the timing checked. Any suggestions that I may not have looked at yet?