1971 Evinrude Mate 2HP outboards Service Manual, Page 17Get this manual
3-2 DESCRIPTION The fuel system consists of fuel tank mounted on the power head fuel filter, and carburetorFuel flows, gravity-fed from the tank'to the carburetorCARBURETORThe 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 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 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 ATMOSPHERIC AIR PRESSURE
The upstroke of the piston in the cylinder creates suction that draws air through the throatA restriction in the throat, call 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 valveTo regulate engine speeds, throttle valve controls the volume of fuelair mixture drawn into the engineTo compensate for the extra amount of fuel required to start cold 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 up, the choke is opened to restore the normal ratio required36177