Carbon film in exhaust?

[natsgrampy]

Ran my boat on muffs yesterday and today found a black "film" in the driveway where the exhaust water puddled. What can cause this and is it a problem in waiting? I have an 89 3.0 Mercruiser. The plugs also have black on them.

 

[Don S]

Re: Carbon film in exhaust?

Normal, comes from the choke being on during starting and warm up.