1978 Johnson 2HP outboards Service Manual, Page 43Get 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 outletSee Figure 6-1
HIGH SPEED
WATER PUMP
EXHAUST RELIEF HOLES
Figure 6-1Exhaust Relief and Swivel Bracket Screws