Re: Evinrude carb gasket with wire mesh opening

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chas56x

US Member - 2 Years
Replies: 49
Topics: 6
#70369

There is no chance of "residual combustion fire" left in the combustion chamber when the piston reaches bottom dead center and the exhaust ports and the intake ports are uncovered. If you look at the position of the intake ports in relation to the exhaust ports, the exhaust ports are higher up in the cylinder bore than the intake ports. The shape of the piston with the deflector on top forces the burnt fuel/air mixture to exit the combustion chamber before the intake ports are uncovered by the piston. By design, when the intake ports are uncovered, pressure in the crankcase forces the intake charge (fuel/air mixture) into the combustion chamber and the deflector on the top of the piston forces the fuel/air mixture up into the top of the chamber and actually forces any left over exhaust gasses out of the exhaust ports. The carburetor in the older Evinrudes are of the poppet type. The poppet valve in the carb moves up and down in relation the pressure/vacuum pulses in the crankcase developed by the up and down movement of the piston. During the downward movement of the piston, pressure builds up in the crankcase forcing the poppet valve in the carb to close with the assistance of a small spring on top of the poppet valve, thus forcing the fuel/air mixture up into the intake side of the cylinder. During the up stroke of the piston, a vacuum is created in the crankcase, unseating the poppet valve allowing a fresh charge of fuel/air mixture into the crankcase.
With the incorporation of reed valves, this negated the need for poppet valve carburetors. If you get any spitting back of fuel/air mixture out of the carburetor in a two cycle engine, it is caused by either a stuck poppet valve or a broken reed.