1980 Johnson 2HP Service Manual, Page 19Get this manual
3-2 DESCRI PTION
ATMOSPHERIC A.JR PRESSURE
The fuel system consists of fuel tank mounted the power head, fuel filter, and carburetorFuel flows, gravity-fed from the tank to the carburetorCARBURETOR The carburetor is single-barrel, f10at feed typeThe high and lowspeed jets are adjustableThe carburetor has manual chokeThrottle is synchronized with the magneto the magneto armature plate
THEORY OF
The carburetor is metering device for fuel and airAt idle speed, an engine requires mixture of about parts air to part fuelspeed mixture is about 12:1small chamber 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 SP EED NEEDLE HIGH SPEED ORIFICE HIGH SPEED OPERATION IDLE AND SLOW SPEED ORIFICES36 177
The upstroke of the piston in the cylinder creates suction that draws air through the throatr estriction in the throat, cal1ed 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 streamHere it mixes with th air to form eombustible mixture for burning in the engine cylindersIn order to mix the fuel and air in just the right proportions for engine speeds, the low speed jet has an adjustable needle valve to pensate for changing atmospheric conditionsThe high speed jet have fixed high speed orifice or ,an adjustable needle valveregulate engine spe eds, throttle valve controls the volume of fuelair mixture drawn into the enginecompensate for the extra amount of fuel requir ed to start engine, choke valve is placed ahead of the metering jets and venturiWhen the valve is closed, very rich fuel mixture is drawn into the engineAs the engine starts and warms the choke is opened to restore the normal ratio equired