1977 Johnson 2HP Outboards Service Manual, Page 19Get this manual
ATMOSPHERIC A,IR PRESSURE
The fuel system consists of fuel mounted the power head, fuel filter, and carburetorFuel f1ows, 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 linkage is synchronized with the magneto the magneto armature plateTHEORY 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 mi.xture 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 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, control1ing air velocityThe differential in throat and chamber air pressures causes the fuel to pushed out of the metering jets and into the air streamHere it mi.xes with the air to form combustible mixture for exploding in the engine cylindersIn order to mi.x the fuel and air in just the right proportions for gine speeds, the low speed jet has adjustable needle valve to pensate for changing atmospheric conditionsThe high speed jet have fixed high speed orifice or an adjustable needle valveregulate engine speeds, throttle valve controls the volume of fuelair mi.xture drawn into the enginecompensate for the extra amount of fuel required to start cold engine, choke valve is placed ahead of the metering jets and venturiVENTURI RING HIGH SPEED ORIFICE NEEDLE