Yamaha F100B F100C Outboards Factory Service Manual, Page 102Get this manual
Power unit
5If the compression pressure is below specification and the compression pressure for each cylinder is unbalanced, add small amount of engine oil to the cylinder, and then check the pressure againNOTE: If the compression pressure increases, check the piston and piston rings for wearReplace if necessary If the compression pressure does not increase, check the valve clearance, valve, valve seat, cylinder sleeve, cylinder head gasket, and cylinder headAdjust or replace if necessary
21Install the cover and flywheel magnet cover
Checking the compression pressure
1Start the engine, warm it up for minutes, and then turn it off2Remove the clip from the engine shut-off switch on the remote control box or tiller handle3Remove the cover and all spark plugs, and then install the special service tools to each spark plug hole
Checking the oil pressure