A Johnson outboard motor's lower unit contains the propeller shaft or, in the case of the jet drive, an impeller shaft. It also houses the drive gears, the driveshaft that comes down from the engine and the water pump that cools the motor. There are slight differences between the method for removing a light duty lower unit, most often found on motors of less than 70 horsepower, and for removing a heavy duty lower unit used on motors of 70 horsepower and up. The method for removing the jet drive is the same, regardless of its horsepower.
Remove the propeller from the propeller shaft. Ensure the gear lever is in the "Neutral" position. Use a pair of needle-nose pliers to remove the cottar pin from the shift rod connector on the front face of the motor, just below the motor's swivel bracket, and discard the pin.
Unscrew the trim tab retaining bolt and remove the trim tab. Remove the seven bolts threaded upward from the lower unit into the motor's midsection with a 5/8-inch socket wrench: three bolts are on each side of the lower unit, and the seventh is the bolt inside the cavity beneath the removed trim tab.
Pull straight down on the lower unit, taking care not to bend the driveshaft or shift rod. The water tube will pull free from the rest of the motor.
Remove the shift rod screw that protrudes from beneath the engine cover. Remove the lower unit bolt that's threaded upward beneath the anticavitation plate in front of the trim tab with a thin-walled, 5/8-inch socket.
Unscrew the two bolts on each side of the lower unit that are threaded upward from the lower unit into the exhaust housing of the motor.
Pull the lower unit straight down, without bending the driveshaft or shift rod or damaging the water tube.
Unscrew all six bolts that secure the intake housing to the jet housing assembly with a 5/8-inch socket. Bend the lock tab washer back from the impeller nut with a pair of pliers. Remove the impeller nut with an adjustable wrench. Remove and discard the lock tab washer.
Remove the impeller and any shims installed below it. Remove the impeller key and sleeve and any shims installed above the impeller. Unsnap the shift link from the reverse gate.
Unscrew the 9/16-inch adaptor-to-jet-housing bolt threaded downward from the adaptor flange to the jet housing. Remove the four bolts underneath the jet drive assembly inside the impeller cavity that are threaded upward into the adaptor. Lower the jet drive assembly from the adaptor.
Keep track of all of the jet drive shims, even if the impeller is being replaced. The shims give a starting point for making adjustments. Also, keep close track of the bolts: these are different lengths and must return to their original mounting holes. Note, too, that the jet drive bolts are outboard from the bearing housing.
Always disconnect the negative cable from the battery before beginning any mechanical work on your motor.