1978 Johnson 55 HP Outboards Service Manual, Page 77Get this manual
DEseR PTION EXHAUST HOUSINGS This motor features "wet sleeve" exhaust system consisting of an inner and an outer housing with water chamber in betweenThe inner housing is loop-tuned unit providing ideal exhaust scavenging throughout the power rangeThe water discharged from the engine cooling system fills the space between the exhaust housings and maintains specified level when the engine is runningSee Figure 6-1The water drains out when the engine is stopped and tilted above the water lineThis chamber of water serves as an effective Silencer, quiets the exhaust relief, and cools the outer housing and lower rubber mountsExhaust gases are discharged underwater through the propeller hub for efficient silenCing and performanceUPPER MOUNTS
PRESSURE VALVE
Figure 6- 2Isolation Mount Arrangements WATER PUMP