When was the last time you changed the plug wires, cap and rotor?
Also...Double check the firing order.

The falling on its face at wot has been going on since I purchassed the boat 2 seasons ago. I replaced the cap rotor button plugs and plug wires last season. This did not effect the issue. Also replaced fuel pump and and check valve and the water seperator twice. Before the last round of changes ie: metering rods plugs and prop I could only pull about 4000 rpms. With the change of these things I was able to achieve 4400 rpm but thst is when the backfire started.

My first thought was plug wires on the backfire issue. On the falling flat, I'm thinking tired valve springs. Don't know what your total hours are but 21 years is an eternity for a boat motor, even sitting. May be time for a top-end job.

A compression test will tell you if you have a problem, but it won't tell you where. You need to do a leakdown before disassembly. No reason to do the heads on a motor with a cylinder problem.

I think in terms of likeliness. The exhaust has probably been leaking and now the valve seats are rusted and pitted. when you changed the spark plugs last season did any have any rust on them. maybe one or two of the center ones? When you do your comp test pay attention to see if there is any signs of rust or water on the plugs.

Chris is right the leak test is more telling but a compression test will help you find direction.

I replaced the plugs last year and this year. All appeared to be normal. Looked almost new when removed. The engine hour meter has been replaced and reads 265. Don't really know why it was replaced if it was due to rebuild or repower. Or could have just been to lower the hours. I really don't know much about what happened to to the boat before I purchased it. What really makes me crazy is that at all other rpms the boat purrs like a kitten. Theres no ticking or anything. I almost wish something would break so I could figure it out.

Check distributer,main engine, and amplifier grounds. Some early motors did not have a ground from dist. housing to motor. Also check main engine plug. Spread prongs and spray with CRC and push in completely. You can try with alligator clips 12 volts to coil.
After you try these things and if compression is good I'd pull valve covers to check springs and valve movement as others have mentioned.
Good luck

For the hell of it...change the coil out.

I've done that more times than I care to remember, and not once was it bad coil. Usually, when someone claims to have fixed a problem by changing out the coil, it's because they changed out the bad connection at the coil.

If I remember correctly 4400 is about the redline for that engine, and with 21 year old springs, you could be getting valve float.

The mercruiser manual suggests 4400-4800rpm @ wot

I had the same problem in a 1989 454 mag at 400 hrs. Went through the same replacement pattern you did. Finally pulled the heads and found a few cracked valves, intake & exhaust.

Compression test did not show the problem. Did not do a leak down though but it might show the problem. In fact the problem got worse. Like you only poped once a while at first above 4000 rpm. Then got worse down to 3500 rpm.

I can't guarantee you have the same problem, but sure seems the same. If you do get the heads done, be sure a shop does it right for a marine engine with Inconel or other correct valves. Good luck.