Loss of power but good idle and starts 1988 OMC 43

"Hi guys, I've been follow

"Hi guys, I've been following this site and it gives me hope of keeping our '88 VIP Cuddy running well. Anyway, here's my problem...
A couple of days ago, I took our 19' cuddy out in the ocean. It started fine, and idled well. After getting bait and heading out to open water, I gave it throttle, and it could only muster up to 10-12 mph (gps). It normally can get up to 28-30 mph with no problem and a full load. Now it would only cruise at 10mph at WOT, and once in a rare while, the power comes back an starts to speed up over 23mph (but I 'd throttle it back down for other reasons). Then once it loses the momentum, it can only do 10mph again. It got 1.3 mpg over a 70 mile trip. Does anyone know what I can check?

Oh yea, one other thing... In neutral, the engine revs up very nicely past 4.5k rpm.. and I dont rev it past that... however, once in gear, the engine stops revving a little past 2k rpm and 10-12 mph. Very confusing.

-I did the these maintenance about a month before the trip: Changed spark plugs (iridium), new spark plug wires, new hoses, T stat, new petcocks for the manifold and engine block, new starter and starter solenoid. The coil is a MSD blaster, (was there when I bought the boat), and assume that it's still good.

-After I got back home I changed the distributor cap, rotor, points and the cylinder unit inside as well. I'm going to check the ignition timing soon, but it hasn't been adjusted since last season.

Please help!"

"I would do a test at each cyl

"I would do a test at each cylinder, while running carefully pull the wire off each spark plug (one at a time). If the cylinder is working you should feel/hear a decrease in RPM's each time, if not then you need to investigate further. I would also do a compression test for each cylinder. Getting up to 4-5k RPM's in neutral is not really telling you anything."

change the fuel filters.
If

change the fuel filters.
If no improvement try going for a run with the ESA module unplugged.
Any excess growth on the hull bottom. If you trailer the boat- does much water come out when pulling the drain plug? Has it rained a lot there? Can you weigh the boat?
As far as revving it up in neutral - that tells you nothing- you could remove half the pistons and still rev it to destruction