1980 Johnson 4HP Service Manual, Page 66Get 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 unrestricted escape for exhaustHowever, in starting, water in the outlet creates back pressure and hard startingExhaust relief is provided another outlet located in the water discharge passage above the water lineSince water is discharged until after the motor is started, the exhaust gases will initially discharged through the water outletWATER PUMP Water for cooling the powerhead is circulated the water pump located at the top of the gearcase and driven directly 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 bIades flex as they rotate, varying the space between them The pump inlet port, located in the stainless steel plate which forms the lower part of the to the bIades when the pump housing, is space between them is increasingThe pump outlet port, in the impeller housing, is to the bIades when the space between them is decreasingThus at low speed the impeller works as displacement pumpSee Figure 6-1