Fouled plug

[ronzoe]

I have a problem with a spark plug that keeps fouling on my 1998 Yamaha 225 EFI's. It is the middle plug on the starboard side of the starboard engine. I have replaced the plug and when I do, the engine runs fine for a few hours, then it fouls again. When the plug fouls, the starboard engine burns about 15-17% more fuel than the port engine. I have already replaced all the spark plugs O2 sensors and all 6 fuel pumps. When the boat is flushed out of the water, the starboard engine leaks a small amout of fuel? out of the prop shaft. I thought the possibility could be the ignition coil, but the mechanic who looked at it said those coils seldom go bad. Any other thoughts. Other than that the engines run fine. PS I do use ring free. Thanks. Ronzoe

[boats.net]

It is probably loosing fire on that cylinder and usually if it's only one cyl. the ign coil, spark plug boots are the problem.