1978 Johnson 4HP outboards Service Manual, Page 72Get this manual
WEED LESS GEARCASE WATER PUMP aRemove "0" ring from top of driveshaftbRemove two impeller housing screwsSee Figure 6-13cWhile hold'ing driveshaft down in gearcase, slide impeller housing and impeller off of driveshaftRemove key and slide impeller plate off of driveshaftRemove and discard water tube grommet from impeller housingdInspectnecessaryand replace components as
Install impeller pin in driveshaftSlide impeller down driveshaft and engage impeller pinLubricate impeller with oil and install impeller housingRotate driveshaft in clockwise direction while slipping pump housing over impeller to position blades correctlyAttach pump housing to gearcase with screws dipped in OMC Gasket Sealing Compound and torque to specificationsfInstall new "0" ring on driveshaftgOil and install new lower water tube grommet in top of impeller housing hReassemble gearcase to exhaust housing as described in "Reassembly of Gearcase to Exhaust Housing