1978 Johnson 4HP outboards Service Manual, Page 19Get this manual
DESCRIPTION FUEL FLOW The fuel system consists of fuel tank, fuel pump, and carburetorThe fuel tank is non-pressurized, suction operatedA diaphragm-displacement type fuel pump on 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 are adjustableThe carburetor has manual chokeThrottle linkage is synchronized with the magneto by cam on the magneto armature plate
dropped to predetermined point on the compression strokeSee Figure 3-1CHOKE The carburetor is fitted with manual choke to reduce the ratio of air to fuel for cold startsA choke valve in the air inlet of the barrel is mounted on the choke shaftWhen the choke knob is pushed in, the valve is held openallowing air to pass freely through the inletWhen the choke knob is pulled out, the valve is closed, restricting the flow of air to the carburetor-