I have a 1992 merc 100hp. the other day i took the boat out. when I hold it wot it goes to about 5100rpms then it will bog down to about 4800rpms then go back up some time it won't go past 4800rpms. what could cause this. I changed my fuel filters blew all the lines out with air. took the carbs apart and they are all clean.and the tank is clean. and rebuild the fuel pump. could this have some thing to do with the timeing. tHANKS FOR ANY HELP