Workshop · spark plug
Reading spark plugs: what they should look like, and what not
Eight spark plug pictures with explanation: what you see, what it means and what to do about it.
The spark plug tells you a lot about your engine. From the colour and deposits you can see whether the mixture is right, whether the engine is burning oil and whether the plug is too hot or too cold. Below you can see what a good plug looks like, and what the other colours mean.
What do you see, and what does it mean?
| Plug | What it looks like | What it means |
|---|---|---|
| Good | Light brown to greyish brown, dry. | Mixture, ignition and plug type are right. Nothing to do. |
| Black soot | Dry, dull black soot on the insulator and electrodes. | The mixture is too rich (choke on too long, dirty air filter, carburettor too rich), the engine is often only run short distances or at idle, or the plug is too cold and does not get hot enough to burn itself clean. First look at the air filter, choke and carburettor. |
| Wet with petrol | Wet and shiny, smells of petrol. | The engine is flooded, or petrol is arriving but there is no spark. See Bike won’t start? and test the spark. |
| Oil | Black, greasy and sticky. | Oil is getting into the combustion chamber: worn piston rings, valve guides or valve stem seals. On a two-stroke there may be too much oil in the mixture. Smoke from the exhaust is often an extra clue. |
| White and glazed | White insulator, sometimes with specks or a glassy layer. | The plug is getting too hot. Causes: too lean a mixture, too advanced ignition, poor cooling or too hot a plug. Do not keep riding like this: too hot can damage the piston. |
| Crusty deposits | Grey, white or yellowish-brown baked-on deposits. | Deposits from oil, poor fuel or the lead from old petrol. Find the cause and fit a new plug. |
| Worn | Rounded electrodes, the gap has grown. | Normal wear. According to NGK the gap grows by about 0.01 to 0.02 mm per 1000 km on a four-stroke and 0.02 to 0.04 mm on a two-stroke. Measure the gap or fit a new one. |
| Cracked | A crack or a chip out of the insulator. | Often from a plug spanner put on at an angle, or from overheating and pinking. Always replace. |
How to read the plug properly
- After a normal ride. Read the plug after a ride of some length, not after just idling or a cold start. Otherwise it soon looks too black.
- Cold engine. Let the engine cool before you unscrew the plug, especially with an aluminium cylinder head.
- Cap off. Pull on the plug cap, not on the lead.
- Unscrew it straight. Use a plug spanner that fits exactly and keep it straight. A spanner at an angle can crack the insulator.
- Good light. Look at the tip in daylight or with a torch, preferably with a magnifying glass.
Measuring the gap and refitting
- Electrode gap. Measure the gap with a feeler gauge. The correct size is in your bike’s workshop manual. Only bend the earth electrode, carefully.
- Clean thread. Clean the thread in the cylinder head and screw the plug in by hand first, until the washer seats. That prevents cross-threading.
- Tighten properly. Tighten it as specified in the workshop manual or on the plug packaging. Too loose and it runs too hot; too tight and you damage the thread and the seal.
- The right type. Use the plug type and heat range from the workshop manual. A colder plug carries away more heat, a hotter one less. Do not just choose a different one.
Good to know
- After reading, change only one thing at a time, and then read again.
- On a twin or multi, read each plug separately: then you can see whether one cylinder is different.
- In doubt? Fit a new plug and look again after a ride.
- For carburettor tuning: see Dell’Orto carburettor. For the ignition: the free degree wheel.
SourcesNGK: How do I read a spark plug? (English)NGK Australia: Spark plug analysis (English)NGK: frequently asked questions (English)