We just put a BRAND NEW Merc. motor in our boat. It starts right up, purrs like a kitten. However, when we accelerate, there's a terrible knocking sound. We haven't run any higher than about 3500-4000 RPMs since it's a new motor and we're breaking it in. When we tried to pull a tuber, the motor was cutting out or missing, and we were not able to pull a tuber or skier. The mechanic first thought it was a problem with a valve, possibly a "floating valve," but checked that out and there seems to be no problem with a valve. Any ideas of what it could be?