And if you are running overly rich - fuel - fuel fouling on the spark plugs like Ron said - MAP, coolant temp, TPS sensor(s), weak ignition components, ,fuel pressure regulator, and finally the injectors. Stuck open and or pulse width too wide could be any of these areas where the problem lies.. Totally agree with the cap and rotor and now a days the coil as well. Coils do not last like they use to, even when properly mounted.

You have to remember that Mercury has to make that engine completely idiot proof. When that engine leaves Merc, they have no idea what half wit, dumb ass is going to get it and put 87 octane in it and run it on the wood. They would rather see you foul a plug than melt it down and them have to pay for it. Now, if you follow their directions and use 91 octane or higher, you can easily run the 7's with much better results (in a stock engine, which the OP has). The same reason applies to why it runs so rich. They have to set it up to where it is pretty much as bullet proof as it can be. Unfortunately, that doesn't always yield the best results. In this case, the 7 is a better choice.
Eddie

Yes you are correct about the octane requirement but we know how people also look at saving costs at the pumps as well. OMC & Volvo marine engines use to recommend 89 octane and most did not followed that either, then you would get the guy in wondering why his motor is having issues with cylinder / piston problems, especially with the crappy gas today. Even E-0 is crappy at the pumps.

You got that right

One other thing to consider. A 500 efi runs 6's. It makes 470 hp at the prop. A 575 makes 525 at the prop. Does going up 50 hp, on basically the same long block, warrant using 2 heat ranges colder? I realize that they have different cams, but the 575 also have 1.25:1 less compression than the 500.
Eddie

All this talk makes me think I should change my wires. They're stock from 2002 with over 400 hours on them. Stock coils and program too. I've changed the cap/rotor/plugs 3 times and just use the stock stuff/gap. Boat still runs the same MPH as when the engines were redone 200 hours ago. It does feel crisper when I put new plugs in. I've had 2 injectors go wide open above idle but only noticed that because of the difference in fuel consumption from L to R engine, er, I mean S to P.
Sounds like a winter project. Woohoo, 110 here I come.

Most already know this, supercharge engine = lower compression ratio and you are comparing Throttle body injection to multi port injection. TBI in a 2000's engine. A little behind in the tech area on that one.

Also forgot the 575 SCi has IAT (intake air temp) sensor that also can cause rich running conditions / spark plug fouling related problems.

Whoa....tell me more about the injector dumping off idle, I am having a similar problem (I think) on starboard right now

Maybe, but the bpw vs IAT enrichment table is so tiny on the ecu mapping I doubt anyone would notice

3.25 8lbs pulleys, full roller conversion, mls gaskets, arp studs, 120 stats, adjustable fuel regulators, and I tuned it using afr and mefi burn.

My stock afr numbers used to get in the low 9s under boost if I left it there. When my engine guy tore them down he suggested that I clean them up because he was seeing dangerous fuel wash. My oil was always black at 20hr oil change intervals stock, now it looks so clean at 20 hrs I want to out it in something else :D

I agree merc was trying to be super safe with this engine, and maybe even for the xr drive. The flat tappet cam was a joke in a gen 6 engine, very restrictive. Stock I would hold it wot for 30 miles and wouldn't even get a rise in oil or water temp. So much life to be found in these engines, and merc really missed with ecu tune IMO. So safe it was nearly unsafe :/

Mine has Ngk Y R5's with s pretty big gap. I am running msd's and chillers. I would hate to see the heat in 4bus motor w o the chillers. But mine never fouled a plug. My buddy said they are too much though. I am just worried about un burnt fuel. Arizona Speed set mine up.