1978 Johnson 2HP outboards Service Manual, Page 19Get this manual
OEseRI PTION
ATMOSPHERIC A.lR PRESSURE
The fuel system consists of fuel tank mounted on the power head, fuel filter, and carburetorFuel flows, gravity-fed from the tank to the carburetorCARBURETOR The carburetor is single-barrel, float feed typeThe high and lowspeed jets are adjustableThe carburetor has manual chokeThrottle linkage is synchronized with' the magneto by cam on the magneto armature plate
THEORY OF OPERATION
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 chamber"HIGH SPEED NEEDLE HIGH SPEED ORIFICE