The spark plug shows you what the mixture was like in the last few minutes before you switched off. It costs nothing and works anywhere. It only helps, though, if you take the plug out at the right moment and know which throttle range it is showing.
How to get a useful reading
If you roll into the garage after a ride through town, you are reading the idle, not the full-throttle run. For a clear picture:
- Fit new or cleaned plugs and warm the engine up.
- Ride the range you want to check steadily for a few kilometres. For the main jet that means full throttle on an open road.
- Pull the clutch, switch off and coast to a stop. Don't ride through the village first.
- On a twin, look at each plug separately. One side can run rich and the other lean.
What the colour means
| Insulator | Meaning | Usually together with |
|---|---|---|
| light brown to greyish brown | correct | engine runs smoothly, no heavy deposits on the electrodes |
| pale, greyish white, as if sandblasted | too lean | weak at the top, pinking, engine runs hot |
| dry sooty black | too rich | burbling, smell of fuel, high consumption |
| wet and oily | not a carburettor issue | oil burning past valve stem seals or piston rings |
A lean reading is serious. Don't keep riding like that, richen the mixture first. With today's unleaded fuel the plug may look a little lighter overall than old manuals describe.
Which part does it point to?
The plug tells you whether the mixture is too rich or too lean. Which part to change depends on how you were riding:
- Reading after full throttle: main jet.
- Reading after steady riding at medium throttle: needle position and needle jet.
- Reading after idling and short trips: idle jet and mixture screw.
The reason is explained in Which carburettor circuit works when.
Where plug reading reaches its limits
A plug shows a direction, not a measurement. Modern fuels colour less, and the difference between "slightly lean" and "correct" is hard to see. For fine-tuning a standard bike it is still the most practical tool, especially when you start from the factory jetting. After bigger modifications a lambda reading or a dyno is the quicker route.
Need jets for testing?
Individual main and idle jets and a main jet set around your factory size are listed on your motorcycle's page in the jetting database. Happy to answer questions.
