Spark plug is wet with oil

The plug comes out wet, black and oily, and smells of oil rather than petrol. Oil is getting into the combustion chamber and burning there. On a four-stroke it comes past worn piston rings, past worn valve guides or valve stem seals, from an overfilled sump, or through the breather into the air box. On a two-stroke, oil is mixed with the fuel by design, so too much oil (wrong premix, oil pump set too high) or a leaking crank seal gives an oily plug. Oily plugs often come with blue smoke.

Is it safe to ride?

With care. Carry a spare plug. Check the oil level often: an engine burning oil can run low. Stop if smoke is heavy or the oil warning light comes on.

Quick checks first

• Is the plug wet with oil (slippery, does not evaporate) or with fuel (evaporates, smells of petrol)? Fuel: see Engine is flooded and smells of fuel.
• Check the oil level: is it above the upper mark?
• Is there oil in the air box or on the air filter?
• Blue smoke from the exhaust? When: on start-up, under load, or when closing the throttle?
• Two-stroke: what premix ratio or oil pump setting is used?

Diagnose it step by step

1Is the oil level above the upper mark?

• Yes: drain to the correct level. The K-Light 125 fault chart lists oil above the upper mark as a smoke cause (PDF page 29). See Too much oil in the engine and Check the engine oil level.
• No: go to step 2.

2Is there oil in the air box or on the air filter?

• Yes: the crankcase breather is passing oil, often a sign of blow-by. Check the crankcase breather.
• No: go to step 3.

3Two-stroke: is the premix or oil pump setting right?

• No: correct it to your manual. Pump faults: Check the two-stroke oil pump and lines.
• Yes, or four-stroke: go to step 4.

4Do a dry and then a wet compression test. Is compression low dry and much higher wet?

• Yes: worn rings or cylinder (MAVCC wet test logic). Test engine compression.
• Compression normal: oil is probably coming past the valve guides or stem seals. Blue smoke after idling or on start-up supports this. Go to step 5.

5Have a workshop confirm with a leak-down test and inspect the top end.

• Do a leak-down test and Assess the top end (cylinder, piston, valves).

Fit a new plug of the correct type after each test so you can see whether the problem returns.

Causes, most likely first

Reference values (K-Light 125)