The Elto Handitwin in question has a "check valve" carburetor. If the cork bumper is worn or damaged so the valve opens wider it will delay closing causing a decrease in efficiency and increase in tendency to backfire. A lean mix burns more slowly so at slower speeds running with retarded spark there will be fire in the combustion chamber when the transfer ports (AKA bypass ports) open which ignites the fuel mix in the crankcase. On rotary valve or third port outboards, this is the time when the intake opens so it can blow a flaim out of the carburetor. The screen is mainly to keep bugs and bits of cork bumper out of the crankcase. For those who feel it necessary, I clean off the old screen completely of any trace of gasket and put a new gasket on both sides. . . 😀
