I have twin 496Mags, and have black on transom... tell me how to fix!

have silent choice but almost never run it, strait tips for the load side. bone stock 496Mags, 160-170 hours on them

I have been running 87 octain in it.

+1 on switching to 87 fuel unles you need the higher octane due to higher compression. I had a 1999 454mag EFI stock and thought i was doing myslef a favor running 93 fuel, transom was always black. switched to 87 and 90% of the soot dissappeared.

Both my Fountains are terrible at this. Just a couple minutes of running and it's black. Tried different fuels, plugs. Nothing.

Having the PCM's reflashed is about the only way.

the engines were rebuilt 2-3 years ago, carbs are rebuilt as of last night. Guess I will run 87 and see what it does from there. I saw the receipt from the rebuild and the cap and wire were replaced so Id have to imagine the the plugs were replaced. I guess it could be a possibility that upon rebuild they set it to run rich and set it back right either. Thanks for all the help.

What do they need to fix with a flash/tune? just the idel mixtures? who in the northern alabama areas does it? how much does it cost?

If this has carbs, what needs to be flashed?

think they were talking the 496

My 496s have always sooted up the transom. Nothing horrible, but noticeable. Just the nature of the beast. Have you tried using a good polish? I use Nautical Ease boat polish and it seems to make the soot wipe off a little easier.

A product called Roll Off. You can get it at West Marine.