I have a 2000 502 EFI w/190 hours. The mystery problem is when the engine is under load while coming on plane it will not rev over 3000 rpm, sputters back to around 2000 to 2500 and will backfire. If I feather the throttle it will eventually come on plane but will only tach around 4500 at WOT. I have replaced Head gaskets and all others that were disturbed, cleaned injectors, new plugs, wires, cap, button, checked fuel preasure, ran scan tool (checked good), checked pcv valve, new fuel filter, and checked all ground wires. If anyone has any suggestions I am open. My local Mercury certified mechanic is even stumped. Trying a check valve for the cool fuel this weekend, hope it works! Also has lightening headers and K/N flame arrestor.

I had more or less the same problem, then after checking everything I found that I had made a mistake with the wire plugs instead having the right firing order, it was that the wires between the #6 and #8 plugs were exchanged.
Harry

Plug a timing light in while advacing the RPM and make sure the timing is advancing - I've had bad ignition modules lock the timing and run like crap...... As if it were in Service mode on the scanner.

When you say you checked the fuel pressure, does that mean you checked it under load, when the engine lays down? Sure you don't have bad fuel?

Thanks for the input will give all of you guys responses a try.( From dboater1)