Johnson Evinrude Outboard Motors 1956-1970 1.5-40hp repair manual., Page 93Get this manual
CARBURETION fuel chamber vent hole is located near the top of the carburetor body to permit atmospheric pressure to act against the fuel in each chamberThis pressure assures an adequate fuel supply to the various operating systems of the engineAir IFuel Mixture suction effect is created each time the piston moves upward in the cylinderThis suction draws air through the throat of the carburetorA restriction in the throat, called venturi, controls air velocity and has the effect of reducing air pressure at this pointThe difference in air pressures at the throat and in the fuel chamber, causes the fuel to be pushed out metering jets extending down into the fuel chamberWhen the fuel leaves the jets, it mixes with the air passing through the venturiThis fuelair mixture should then be in the proper proportion for burning in the cylinder s for maximum engine performanceIn order to obtain the proper airfuel mixture for all engine speeds, high- and lowspeed needle valves are installedOn latemodel engines, the high-speed needle valve was replaced with high-speed orificeThere is no adjustment with the orifice typeThese needle valves are used to compensate for changing atmospheric conditionsProbably over 50 of the engines covered in this manual have an adjustable high- and low-speed needle valve 4-3