A spark tester lets you see the spark outside the engine, clipped to bare metal, with the plug cap on. It takes a few minutes to make from a spare plug. Use it with Check for a spark safely.
Safety
- The spark can ignite fuel vapour. Keep petrol, spilt fuel and rags away from the tester. Test outdoors or in good air. Keep a fire extinguisher close.
- The coil makes thousands of volts. Do not hold the plug cap, the tester or the lead in your hand while cranking. Hold insulated parts only.
- Never pull the plug cap off a running engine. The coil has to push its voltage somewhere, and it can damage the ignition unit.
You need
- A new or good spare spark plug of the correct type for the bike.
- A short wire and a small clip.
- Pliers and a way to join the wire to the plug (solder or a crimp).
Steps
- Hold the plug with pliers and bend the side (earth) electrode out, away from the centre electrode, so the gap can be opened wide. Do not use the plug in an engine again afterwards.
- Join a short wire to the metal body of the plug (the hex, not the thread tip). Solder it, or crimp it to a ring. Put a clip on the other end.
- Insulate the joint with tape or heat-shrink.
- In use: fit the plug cap to the plug. Clip the wire to bare metal on the cylinder head, away from fuel lines and carb.
- Key on, engine stop switch on RUN, neutral. Crank the engine. Watch the end of the plug in shade: a firm, bright, blue-white spark is good.
- For a fair test, open the gap to about 6 mm (1/4 inch) with the electrode bent out. A good ignition jumps this gap in free air. Less than that in free air is a weak spark, because a spark needs more voltage to jump a gap inside the engine under compression than outside it.
- Electronic ignition: the engine must turn fast enough. Slow cranking on a weak battery can stop the spark. Charge or replace the battery before you decide the ignition has failed.
Check your work
- The tester sparks every turn at the gap you set, or you know the ignition is weak.
- The wire is clipped to clean metal.
- You have removed the tester and refitted the correct plug.
If it did not fix the problem
- No spark: No spark at the spark plug.
- Weak or orange spark: Weak, thin or orange spark.
- Engine cranks slowly: Engine cranks slowly.