soot film at engine start

[natsgrampy]

Got the boat on the water and noticed something odd.
When it started, it created a black film of soot around the exhaust. It was about 1 foot by 3 feet and floated away.
It didn't look like it was oil or gasoline but wanted to ask here to be sure. There was also black smoke at startup too.
1989 searay 200 with a mercruiser 3.0.

Thanks for all you help thus far,

 

[stonyloam]

Just running rich until the engine warms up and the choke opens. Not unusual on cold startup.