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I ride a Piaggio scooter with a single cylinder engine.

Recently I broke down and the engine wouldn't restart. It turned out the spark plug was stuffed.

How can I ensure that doesn't happen again?

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Post a pic of your spark plug and we can help you diagnose the cause. –  Jay Bazuzi Sep 20 '11 at 17:45
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1 Answer

up vote 14 down vote accepted

Your owners manual should tell you how often to replace the spark plugs in addition to that I would inspect the spark plugs every 5000 miles.

Spark Plug Conditions

  1. Normal operation will show a light tan or gray color. The gap clearance will be slight with very little deposits on the insulator tip.
  2. A plug that indicates replacement will show electrode and ground eroded away. If this condition is general on all plugs in the engine, check for sticking valves, faulty ignition leads, breaker points or weak coil or condenser. These conditions usually mean that the next hotter plug should be used.
  3. Oil fouling is indicated by oily, black, sludgy deposit on the plug. A hotter plug would be recommended but will not replace a needed overhaul.
  4. Splashed fouling is applied to plugs that have splotchy deposits on the insulator. These deposits have accumulated through misfiring or inefficient operation. Replacement and tuneup would be recommended for proper performance.
  5. Core bridging or gap bridging is caused by materials of combustion lodging between electrode and the ground, causing the plug to short out. Excessive deposits are most common when oil control is poor or when stop and start operation is prevalent.
  6. Overheating of a plug is characterized by a white or yellow glaze, a burned or blistered insulator nose and badly eroded electrodes. This may be caused by faulty thermostat, correcting engine timing, plug heat range too hot or carburetor set too rich.

Source

Spark plug chart

Link to full size image

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