By examining spark plugs, you can learn a lot about whats going on in a gas engine, whether its an inboard Atomic 4 or your dinghy outboard. Monitoring the look and smell of your exhaust and paying attention to the sounds you hear underway are the best everyday barometers, but the state of your plugs when you change them is revealing.
– Oily plugs.These are your best indicators that something deserves attention. If the plugs are wet with oil it means that oil is entering the combustion chamber. The blow by could come from worn piston rings, valve seats, or perhaps the head gasket. It might also point the finger at faulty timing. In a two-stroke engine, oily residue can indicate a fuel mixture that contains too much oil. An oily plug is a sign to heed.
– Burned, pitted plugs.The insulator tip is gray, and you may see other signs of heat deformation. The engine has been running too hot due, perhaps, to a too-lean fuel mixture. Also check your cooling system, especially the thermostat if the engine has one.
– Normal plugs. If the plugs before you are dry and free from carbon deposits, and if the insulator tips are tinged with tan or slightly brown and there is very little pitting of the electrodes, it is a sign that your engine is running well.
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