1971 Johnson 60HP outboards Service Manual, Page 19Get this manual
3-2 .DESCRIPTION VENTURI INDUCED LOW PRESSURE
FUEL FLOW The 60 HP fuel system consists of fuel tank, fuel pump, and three carburetorsThe fuel tank is non-pressurized, suction operatedA diaphragm -displacement type fuel pump on the motor draws fuel from the tank and feeds it to the carburetors through fuel filterCARBURETORS Each carburetor is single barrel, float feed typeThe high-speed jet is fixed and only the low-speed needle is adjustableThrottle linkage is synchronized with the distributor by cam
ATMOSPHERIC AIR PRESSURE
THEORY OF OPERATION The carburetor is metering device for mlxmg 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 valve