I have a mid 80s Mariner 50 hp Outboard. It wasn't getting spark at the 3 cylinder and the spark plug was black when I removed it. I put a new spark plug in and the same thing happened 'no spark' and black after running for a short period of time. I put in a new coil and got spark. But still the motor ran like it was missing. pulled #3 cylinder plug and it was black again. Any ideas?