1971 Johnson 4HP Outboard Motors Service Manual, Page 15Get this manual
DESCRIPTION FUEL FLOW The fuel system consists fuel tank, fuel and carburetorThe fuel tank is non-pressurized, suction operateddiaphragm-displacement type fuel the motor draws fuel from the tank and furnishes it to the carburetor through fuel filterCARBURETOR The carburetor is single-barrel, float feed typeThe high-speed and low-speed jets adjustableThe carburetor has manual chokeThrottle linkage is synchronized with the magl1eto the magneto armature plateINDUGED LOW PRESSURE VENTURI
THEORY OF OPERATION
The carburetor is metering device for mixing fuel and airAt idle speedan engine requires mj,xture of about parts air to part fuelHigh speed mixture is about 12: 1small chamber holds th!: fuelfloat valve admits fuel from the fuel tank to replace fuel as it is cOl1sumed the engineMetering jets in the buretor throat extend down il1to the fuel chamberGASE
ATMOSPHERIG AIR PRESSURE
THROTT LE VALVE
IDLE AND SLOW SPEED ORIFIGES
AIR INTAKE