2007-2009 Yamaha F15/F20 Outboard Service Manual, Page 91Get this manual
Power unit Minimum compression pressure (reference data): 730 kPa (7.3 kgfcm2, 105.9 psi) (Manual starter model) 860 kPa (8.6 kgfcm2, 124.7 psi) (Electric starter model) 6If the compression pressure is below specification or the compression pressure for each cylinder is unbalanced, add small amount of engine oil to the cylinder, and then measure the pressure againNOTE: If the compression pressure increases, check the pistons and piston ringsReplace if worn If the compression pressure does not increase, check the valve clearances, valves, valve seats, cylinder sleeves, cylinder head gasket, and cylinder headAdjust or replace if necessary
Power unit (check and adjustment)
Checking the compression pressure
1Start the engine, warm it up, and then turn it off2Remove the clip from the engine stop lanyard switch3Disconnect the spark plug caps, remove the spark plugs, and then install the special service tool into spark plug hole