1978 Johnson 175, 200, 235 HP Outboard Service Manual, Page 38Get this manual
FUEL TANK DESCRIPTION The fuel tank supplied with the motor is of Simple but rugged construction, with capacity of gallons (22.7 litres) of fuel mixtureIt contains the bulb primer (for priming the fuel pumps) fuel hose and connector, bracket arrangement to hold fuel line when not in use, carry grip, and manual valve which prevents any escape of gaSOline or fumes, minimizing the danger of explosion or fireSee Figure 3-40
F UEL MIXTURE motor in excellent mechanical and operating condition may give faulty performance because of an improper fuel mixturePetroleum gum and varnish which precipitate from stale mixture may clog the filter screen and any small orifices, interfering with starting and normal runningFor proper fuel mixture, see Section 2, "Engine Fuel and Lubricant 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 mixture