1984-1986 Mercury Force 9.9 and 15HP Outboards Service Manual, Page 45Get this manual
A.INTAKE Inlet port is uncovered, fuel mixture enters combustion chamberFlow pattern 01 fresh tuel mixture helps push out exhaust
BCOMPRESSION Intake and exhaust ports are covered by pistonPressure increases In combustion chamber and decreases in crank case allowing fresh fuel mixture to enter through reeds