1972 Johnson 4HP Outboard Motor Service Manual, Page 15Get this manual
DESCRIPTION FUEL FLOW The fuel system consists of 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 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 magneto the magneto armature plateINDUGED LOW PRESSURE VENTURI
THEORV OF OPERATION
The carburetor is metering device for mixing fuel and airAt idle speed; an engine requires mixture of about parts air to part fuelspeed mixture is 12:1small holds the fuelfloat valve admits fuel from the fuel tank to replace fuel as it is consumed the engineMetering jets in the buretor throat extend down into the fuel chamberThe upstroke of the piston in the cylinder creates suction that draws air through the throatrestriction in the throat, called venturi, has the effect of reducing air pressure at this point, controlling air velocityThe differential in throat and chamber air pressures causes the fuel to pushed out of the metering jets and into the air streamit mixes with the air to form combustible mixture for exploding in the engine cylindersIn order to mix the fuel and air in just the right portions for engine speeds, the high and low speed jets have adjustable needle valves to compensate for changing atmospheric conditions