2001 Edition Yamaha F225A and LF225A Outboards Service Manual, Page 17Get this manual
Features and benefits Exhaust passage during idle To reduce noise when the engine is idling, the exhaust gas passage of the F225 has adopted labyrinth constructionDuring idling, the exhaust gas passes through the passage on the side of the exhaust guide, and enters the upper case through hole in the upper case gasketWhen it fills the upper case, the exhaust gas enters another passage on the side of the exhaust guide through another hole in the upper case gasketThen, it is discharged through the idle port that is provided in the upper caseThe exhaust manifold and muffler in the upper case are surrounded by the water jackets to reduce exhaust noiseThe water jackets also help prevent corrosion by preventing the exterior of the exhaust manifold and muffler from coming in direct contact with the exhaust gas