1972 Johnson 2HP Outboard Motor Service Manual, Page 40Get this manual
6-2 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 another outlet located in the water discharge passage above the water 1ineSince water is discharged until after the motor is started, the exhaust gases will initially discharged through the water out1etSee Figure 6-1WATER PUMP
EXHAUST RELIEF HOLES
Figure 6-1Exhaust Relief and Swive1 Bracket Screws
IMPELLER
Water for cooling the" power head is circu1ated the water 10cated at the top of the gearcase and driven direct1y 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 driveshaftBecause the housing is offset, the impeller blades flex as they rotate, varying the space between themThe in1et port, 10cated in the lower part of the housing, is open to the blades when the space between them is increasingThe out1et port, in the impeller housing, is open to the blades when the space tween them is decreasingThus the impeller works as disp1acement 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 as centrifuga1 Water is conducted from the in the 10wer unit to the power head through copper tube in the exhaust housingSee Figure 6-2REMOVAL OF GEARCASE