I have done everything that I can to figure out what the problem is. My boat has a rebuilt chevy 350 in it. It will get on step but I cannot get it to go over 3,000 rpms, it will fall falt on its face. I have replaced the starter, alternator, and coil. I have checked the timing and adjusted it to book specifications. I have not replaced the distributor but it is the last thing that I can think of that might be causing the problem. Fuel is fine, fuel filter is new. What can it be?