Instruction manual

PROFICIENCY IN SURVIVAL CRAFT AND RESCUE BOATS OTHER THAN
FAST RESCUE BOATS
9 4
Fig.9.5 The outboard motor cooling system
9.5 Internal lubrication system
Two-stroke engines need their crankcase for compression, therefore, a
circulating oil system is not possible. Their lubrication is achieved by mixing oil
with petrol in one of two systems. Oil is either poured into the fuel tank and mixed
by shaking or mixed as required by an oil injection pump known as the “Variable
Ratio Oiling” (VRO). Modern two-stroke engines are usually fitted with such
precision blend systems. Diesel and four-stroke petrol engines are lubricated by
oil circulation from the sump (crankcase).
Insufficient oil will cause the engine to overheat and eventually seize.
Excessive oil will foul the spark plugs, cause smoky exhaust and heavy carbon
deposits. Both will cause engine failure.
The required amount of oil per litre of petrol varies between 1:100 and
2:100. The manufacturer’s recommendations should be followed. Paint the
correct oil/petrol ratio on the fuel tank and use only the recommended two-stroke
outboard motor oil.
With oil injection systems, the VRO pump automatically adjusts and mixes
the amount of oil required by the engine under different conditions and speeds. It