1968 Evinrude Starflite 100 HP outboards Service Manual, Page 16Get this manual
INDUCED LOW
VENTURI
DESCRIPTION
FUEL FLOW The fuel system consists of fuel tank, fuel pump, and carburetorThe fuel tank is non-pressurized, suction operatedA diaphragm-displacement type fuel pump on the motor draws fuel from the tank and furnishes it to the carburetor through fuel filterCARBURETOR The carburetor is four-barrel, side-draft type with straight-in manifolds to each cylinderHigh-speed jets are fixedSeparate low-speed jets for each barrel are linked together for simultaneous adjustmentA separate float bowl and needle valve control fuel to each barrelATMOSPHERIC AIR PRESSURE
THROTTLE
THEORY 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 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 jets have an adjustable needle valve to compensate for changing atmospheric conditionsThe high speed jet may have fixed high speed orifice or an adjustable needle valveFLOAT