Exuast issue

[Blackmax]

Hi
my Family complain of smoke at the back of the boat. smoke comes out from the square hole below engine.
the engine is mercury 135 hp 1990

 

[flyingscott]

normal

 

[RickM46]

The square hole you are referring to (I think) is the Relief Exhaust Port; it comes into play to allow the engine to idle since at idle there is not enough exhaust gas pressure to push past the water in the lower unit and go out the propeller hub; so this opening gives the exhaust a place to exit. I discovered this in 1995 when my '92 Merc 40 2 stroke would not idle because the relief exhaust ports were clogged with carbon.