My boat at wot spins from 5,000 to 5,100 when running strong. About half the time after holding the throttle down for about 8 to 10 seconds the rpms drop to about 4,700 rpms. It does not happen all the time but does happen about half the time. Any suggestions? It is an enclosed motor with a small air cleaner. The motor is a 540 with an 850 carb. Any suggestions would be appreciated. I can provide more details if needed.