1975 Johnson 4HP 4R75, 4W75 Outboards Service Manual, Page 47Get this manual
DESCRIPTION EXHAUST RELIEF Normally, exhaust gases are conducted down through the exhaust housing and out the underwater outlet in the gearcaseThe siphoning action of the propeller and water provides an unrestricted escape for exhaustHowever, in starting, water in the outlet creates back pressure and hard startingExhaust relief is provided by another outlet located in the water discharge passage above the water lineSince no water is discharged until after the motor is started, the exhaust gases will initially be discharged through the water outletWATER PUMP Water for cooling the power head is circulated by the water pump located at the top of the gearcase and driven directly by the driveshaftThe pump consists of synthetic rubber impeller which is keyed to the driveshaft, and the pump housing which is offset from the center with respect to the driveshaftBecause the housing is offset, the impeller blades flex as they rotate, varying the space between themThe pump inlet port, located in the stainless steel plate which forms the lower part of the pump housing, is open to the blades when the space between them is increasingThe pump outlet port, in the impeller housing, is open to the blades when the space between them is decreasingThus at low speed the impeller works as displacement pumpSee Figure 6-1Figure 6-2Swivel Bracket