1972 Johnson 4HP Outboard Motor Service Manual, Page 46Get this manual
DESCRIPTION EXHAUST RELIEF Normal1yexhaust gases are conducted down through the exhaust housing and out the underwater outlet in the gearcasesiphoning action of the propel1er and water provides an unrestricted escape for haustHoweverin starting, water in the outlet creates back pressure and hard startingExhaust relief is provlded another outlet located in the water discharge passage above the water lineSince water is discharged until after the motor is started, the exhaust gases initially discharged through the water outletWATER PUMP Water for cooling the power head is circulated the water located at the top of the gearcase and driven directly the di'iveshaftconsists of synthetic rubber impel1er which is keyed to the driveshaft, and the housing which is offset from the center with respect to the driveshaftBecause the housing is offset, the impel1er blades flex as they rotate, varying the space between theminlet port, located in the stainless steel plate which forms the lower part of the housing, is open to the blades when the space between them is increasingoutlet port, in the impel1er housing, is open to the blades when the space between them is decreasingThus at low speed the impel1er works as displacement See Figure 6-1Figure 6-2Swivel Bracket Remove the four screws attaching the lower tor cover to the exhausthousingLift the power head from the exhaust housingInner exhaust housing is attached to the lower motor cover and power head with four screwsWater tube is attached to the lower motor cover with compression sleeve and nutSee Section 5, Figures 5-3 and 5-4