1975 Johnson 4HP 4R75, 4W75 Outboards Service Manual, Page 16Get 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 plateINDUCED LOW PRESSURE VENTURI
THEORY OF OPERATION
ATMOSPHERIC AIR PRESSURE
The carburetor is metering device for mixing fuel and airAt idle speed, an engine requires mixture of about parts air to part fuelHigh speed mixture is about 12: 1A small chamber holds the fuelA float valve admits fuel from the fuel tank to replace fuel as it is consumed by the engineMetering jets in the carburetor throat extend down into the fuel chamberThe upstroke of the piston in the cylinder creates suction that draws air through the throatA restriction in the throat, called venturi, has the of reducing air pressure at this point, by controfling air velocityThe differential in throat and chamber air pressures causes the fuel to be pushed out of the metering jets and into the air streamHere it 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 proportions for all engine speeds, the high and low speed jets have adjustable needle valves to compensate for changing atmospheric conditionsTHROTTLE IDLE AND SLOW SPEED AIR INTAKE