How do you change an impeller on a Johnson outboard?

remove the lower unit of the motor (the part with the prop attached to it) by loosening the four bolts which hold it on, as you are doing this be carefull to hold the lower unit in place when loosening the last bolt so it doesn't fall to the ground. Inside the unit is a small box held on by four bolts, this is the water pump, remove the four bolts and the gaskets (there are three gaskets two rubber gaskets which sandwich a metal gasket). The small turbine lookin thingy is the impeller. Remove the impeller inspect the casing and gaskets for cracks or damage. If the water pump housing is cracked or damaged replace it same with the gaskets. Install new impeller and repeat thes steps in reverse order to re-assemble.