After removing my flame arrester to clean it I was surprised when I noticed a heavy deposit of black tar on the throttle body plate. There was also a smaller amount on the inside when I opened the throttle and stuck my hand in. It's a 2004 496 mag HO with 190 hours. I can imagine there's some of that now on the valves and throughout the intake manifold.

Does anyone else use a engine cleaner like the Mercury power tune on this engine? I used to use it on my old carbed 454 now and then but I'm just not sure how the EFI will like it. It's nasty stuff.

It is belt dust combined with moisture, oil and whatever else...

Replace the belt, tension it correctly and it will decrease dust. There are a few threads talking about this. you may want to clean your pullies with a wire brush before the new belt goes on. Some 496's have more dust... I think some pullies arent aligned as well as others.

The TB comes off with 3 nuts. It might be a good idea to pull it and clean it. Or, take your time with a rag and some throttle body cleaner. (any cleaner, carb, brake, laquer thinner)

That's really interesting. I think you're right. I just replaced the belt last season and I've been having a lot of belt noise since.

I replaced the 3 idler pullys thinking the noise was from there but I discovered it's the new belt. I had crossed referenced it at NAPA so it's not a Mercrusier belt.

Thanks!

Yeah, that is probably the issue.

Does it have the idler pulley for tension or the adjustable alternator bracket?

adjustable alternator

I ordered a Mercursier belt and a K&N filter today. At least the filter will keep the junk out.

The belt tension is tighter than what you think. Very little deflection. Clean the pullies with a wire brush.