1970 Johnson 115 HP Outboard Motor Service manual, Page 68Get this manual
DESCRIPTION
EXHAUST HOUSINGS
The lower unit feature "wet sleeve" exhaust syste consisting of inner and an outer housing with water chamber in betweenThe water discharged from the engine cooling system fills the space between the exhaust housings and maintains specified leve when the engine is runningSee Figure 6-1The water drains out when the engine is stoppedT his chamber of water serves as an effective ile ncerquiets the exhaust elief, and cools the outer housing and lowe rubber
Exhaust gases are discharged underwate through the propelle hub for efficient ilencing and performanceWate inlets in the trim tab scoop up water and direct it back into the exhaust chamber to cool exhaust gases through the propellerEXHAUST RELIEF
No rmally exhaust gases conducted down through the inne exhaust tube and out through the underwater exhaust outletHowever, in starting, water in the underwater outlet creates back pressureThis can cause hard tartingExhaust relie! is ovided by an outlet passage above the waterlineThe exhaust gases will InitiaUy be discharged through the exhaust relief outletSee Figure 6- 1RUBBE M:)UNTS