1979 Johnson 2HP Outboards Service Manual, Page 19Get this manual
INDUCED LOW PRESSURE
-ATMOSPHERIC AIR PRESSURE
fuel system consists of fuel tank mounted the power head, fuel filter, and carburetorFuel flows, gravity-fed from the tank to the carburetorCARBURETOR carburetor is single-barrel, float feed typehigh and lowspeed jets adjustablecarburetor has manual chokeThrottle linkage is synchronized with the magneto the magneto ature plate
THEORY OF
carburetor is metering device for mixing fuel and airAt idle speed, an engine requires mixture of about parts air to part fuelspeed mixture is about 12:1small holds the fuelfloat valve admits fuel from the fuel tank to replace fuel as it is consumed the engineMetering jets in the carburetor throat extend down into the fuel chamber"HIGH SPEED NEEDLE HIGH SPEED ORIFICE HIGH SP EED OPERATION IDLE AND SLOW SPE ED ORIFICES VAL E
The 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 pointThe 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 cylindersenIn order to mix the fuel and air in just the right proportions for gine speeds, the low speed jet has an adjustable needle valve to pensate for changing atmospheric conditionsThe high speed jet have fixed high speed an adjustable needle valve