1989 Johnson Evinrude "CE" 9.9 thru 30 Service Manual, P/N 507754, Page 64Get this manual
Fuel Tank Servicing
Drain and flush the tank at least once year, and at every tune-up or major repairDrain tank at an outdoor location, away from all possible sources of ignitionA drain screw is provided in the fuel tank upper housingShake the tank before draining to mix sedimentsClean the tank by flushing with clear gasoline or solventPrimer pump, screens, etc may be inspected and cleaned as described belowRefill the tank with the correct fuel mixture
Cleaning and Inspection If the tank or hose appears to be leaking fuel or fumes, pressure test tank and hose assembly before doing any disassemblyClean the fuel tank and inspect for rust or leakageReplace tank if badly rusted or if it has developed leaks in the metal body