Welcome to the forum. We'll let some of our BBC experts like Pat, Dave, and others give you a professional opinion, but my first thoughts are that the valve springs are just tired. Your valves might also be tuliping, sucking into the heads a little. Boat engines run at 4500 or so rpms all day long, and that is like taking your car and going down the expressway at 100 mph hour after hour. Boat motors age much faster than car engines because of that fact.

Now we'll see what the guys who really know have to say.

Don