1975 Johnson 4HP 4R75, 4W75 Outboards Service Manual, Page 24Get this manual
To assure that the fuel tank contains the proper mixture, drain and flush the tank at least once year, and at every tune-up or major repairClean the tank by flushing with clear gasoline or solventPrimer, screen, etc may be inspected and cleaned as described belowRefill the tank with the correct fuel mixtureDESCRIPTION The fuel tank is of Simple but rugged construction, with capacity of gallons of fuel mixtureIt contains the bulb primer (for priming the fuel pump), fuel hose and connector, bracket arrangement to hold fuel line when not in use, carry grip, and disc valve which prevents any escape of gasoline or fumes, minimizing the danger of explosion or fireSee Figure 3-18B UL RIMER DISC
from escapingalternately f:Jiow should enter but be serviced by 3-20
To check operation of the valve, into the valve and draw outAir not escapeValve components can unscrewing from tankSee Figure
Figure 3-20
Fuel Tank Vent Assembly