1976 Johnson 2HP 2R76 Outboard Motor Service Manual, Page 19Get this manual
3-2 OEseRI PTION
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 type The high and lowspeed jets are adjustableThe carburetor has manual chokeThrottle linkage is synchronized with the magneto by cam on the magneto armature plateTHEORY 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 effect of reducing air pressure at this point, by controlling 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 low speed jet has an adjustable needle valve to compensate for changing atmospheric conditionsThe high speed jet may have fixed high speed orifice or an adjustable needle valveSVpAELEVDE NOFZLZOLAET CHAMBER FLOA VENTURI RINGHIGH SPEED ORIFICE HIGH SPEED NEEDLE