Free printableTroubleshootingTSP-TS-09
Scooter smoking: what blue, white and black each mean
Three colours, three quite different faults. And on a two-stroke, some smoke is meant to be there.
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TSP-TS-09 · version v1.0 · September 2026. Opens in your browser's PDF viewer, where you can print directly.
The problem it solves
Smoke is the most visible fault an engine can have and the most commonly misread. The three colours have three unrelated causes, when it appears matters as much as what colour it is, and on a two-stroke a certain amount is simply the machine working as designed.
Before anything else: is it a two-stroke? If so it burns oil deliberately, mixed with the fuel or injected, and blue-ish smoke on start-up and at idle is normal. What is not normal is a change — more than it used to make, or smoke that persists once warm.
On a four-stroke the colours separate cleanly. Blue is oil getting into the combustion chamber. White that persists once warm is coolant, on the machines that have any. Black is unburnt fuel, which means the mixture, not the engine.
When it smokes narrows it further still. Only on start-up points at valve stem seals letting oil down overnight. Only under acceleration points at rings. Constantly points at something more serious than either.
The mistakes this sheet exists to prevent
- Diagnosing a two-stroke's normal exhaust smoke as a fault.
- Confusing condensation on a cold morning with white smoke. Steam disappears; coolant smoke does not.
- Reading black smoke as an engine fault. It is a mixture fault, and the mixture table finds it.
- Ignoring blue smoke because the engine still runs. Oil consumption is a wear measurement.
- Not noting when the smoke appears, which narrows the cause more than the colour does.
Who it is for
- Anyone whose machine has started smoking, or is smoking more than it did
- Buyers watching a cold start and unsure what they are seeing
- Anyone told their engine is finished on the strength of some smoke
What is on it
- The three colours and what each means on a four-stroke
- What is normal on a two-stroke, and what is not
- When it smokes, which narrows it further than colour alone
- An oil consumption record, because blue smoke is a measurement
How to use it
- 01Establish first whether the engine is a two-stroke. It changes everything on this sheet.
- 02Note the colour in daylight, against a light background if you can.
- 03Note exactly when it appears: cold start only, under acceleration, on the overrun, or constantly.
- 04Start measuring oil consumption. It turns an impression into a number.
Use it with the registry
Other sheets you may want
- Chinese scooter overheating and running hotAir-cooled engines depend on shrouding and oil more than owners expect. A relative test, because the published temperature figures contradict each other.
- Scooter compression test logOne reading tells you little. Four over a year tell you whether the engine is going backwards, and how fast.
- Spark plug record and reading logThe plug is the cheapest diagnostic instrument on the machine. A record of what it looked like each time turns it into a trend rather than a guess.
Free to print, copy, and share — including in forums, clubs and workshops. If you find a mistake on this sheet, tell us and it gets corrected. Quote TSP-TS-09 and the version so we know exactly which sheet you mean. All printables.