1979 Johnson 2HP Outboards Service Manual, Page 43Get this manual
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 for exhaustHowever, in starting, water in the outlet creates back pressure and hard startingExhaust relief is provided another outlet 10cated 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 outletSee Figure 6-1WATER PUMP
EXHAUST RELIEF HOLES
Figure 6-1Exhaust Relief and Swivel Bracket Screws
HIGH SPEED LOWSPEED
Water for cooling the power head is circulated the water 10cated at the top of the gearcase and driven directly the driveshaftThe consists of synthetic rubber impeller which is keyed to the driveshaft, and the housing which is offset from the center with respect to the driveshaftthe housing is offset, the impeller blades flex they rotate, varying the between themThe inlet port, 10cated in the 10wer part of the housing, is to the blades when the between them is increasingThe outlet port, in the impeller housing, is to the blades when the tween them is decreasingThus the impeller works displacement See Figure 6-2Water is conducted from the in the 10wer unit to the power head through copper tube in the exhaust housingAt higher speeds, water resistance keeps the blades from contact with the housing and the acts centrifugal Water is ducted from the in the 10wer unit to the power head through copper tube in the exhaust housingSee Figure 6-2