MILD THUNDER

I have twin 420 mercs. The port engine has a problem. When I chop the throttle on the starboard, it sounds tough, like a 420 should. When I chop the port throttle it sounds half as loud. The engine seems to idle smooth, but under load it misses, and doesnt have any real power. When running, my engine synch gauge shows that motor down on power. Also, while running it at 1500 rpm under load, the port engine sounds like a vaccuum cleaner when I have the hatch open. It sounds liek a major vacuum leak, but I dont have a vaccum leak. So far, I replaced carbs, fuel filter, distributor, plugs and wires, coil, etc.

Im thinking
Valve Springs broke/worn,
cam lobe going flat
damaged valves

If it were a flat cam or bent valve, I would think it would backfire out the exhaust constantly. it only popped out the exhaust once or twice. It just has no power, and the actual exhaust tone is quiet compared to the healthy motor. What am I missing?