1984-1986 Mercury Force 9.9 and 15HP Outboards Service Manual, Page 44Get this manual
SECTION
POWER HEAD
GENERAL
An engine needs three ingredien ts to runproper fuel air mixture, com pression and spark at the right timeThe information in this section is concerned with compression and the parts of the power head that develop compression
When the piston is moving upward the cyl inder the crankcase is lowering in pressure and allowing fuel to aga in enter the crankcaseRefer to Figure 1-B