Hi, I have a Mercury 40 hp Classic 4 cylinder Oil Injected outboard serial no: OD229057 Brunswick Corp. Fon Du LAC, Wis. USA. It is a 1993. The trouble I am having is a small amount of water is entering the bottom cylinder. After a short time of running the motor the bottom spark plug stops firing due to water shorting it out. The motor has never been in salt water so I ruled out corrosion through the cylinder wall. The engine looks like new, the paint is not discolored so I ruled out over heating. All the cylinders have 125 psi. It came off a pontoon boat. What else could be the problem? Please help save my families camping trip this summer by giving me a hint of what to look for God Bless!
Will replace suggested parts and post my results!