The savvy scooterist can tell a lot about the health of their scooter's engine by examining the "business end" of their spark plug. The color and composition of deposits on the electrode and porcelain insulator indicate whether your engine is running as designed, or is in need of repair or adjustment.

It is important to remember however, that these visual indicators will only show the most recent operating conditions. A plug pulled from an engine that has been left idling may not appear the same as a plug pulled from an engine that was last run at maximum speed. In other words; it is best to check your spark plug after first run your scooter's engine under a full throttle, and them immediately stopping the engine and pulling the plug.

Good
A spark plug pulled from an engine that is in good running condition will normally display a light coat of fine tan or light gray residue on the electrode.

Too hot
If your engine is running too hot, your spark plug's insulator tip may show a glossy, melted appearance. Additionally, excessive heat can leave the porcelain insulation around the electrode looking porous. A powdery white coating may be saying that your engine is running hot due to a too lean fuel/oil mix.

Too cold
A scooter engine that labors below optimum running temperature will often cause the spark plug's tip to have heavy deposits of uncombusted fouling.

Bad gas or oil mix
A wet oily black deposit on the spark plug is a clue that there is a problem with the fuel mix, or that there is too rich an oil/gas mixture in a 2-stroke engine. A mix that is too lean may leave a white residue.

Worn out
A spark plug that has given good service, but is at the end of its life cycle will show a wider gap due to natural erosion, as well as increased fine tan or light gray residue.