I have a 1978 Johnson 15 Horse that I bought to use as a kicker motor on my walleye boat. I cleaned the carb because I was having trouble starting it. After the cleaning it starts fine. I used it for the first time yesterday for trolling. It runs good except once in a while it dies. Starts right up and runs for a while and dies again. When I got back to the boat launch I noticed there were two black streaks coming out of the exhausts and down the motor. Black junk all over the place, including the back of the boat. What would cause this? Black exhaust from an auto motor usually means it is running rich. Any help is appreciated

Jeff Nix