When diagnosing engine stumbling or misfiring, its often useful to gauge the intensity of the spark at each cylinder. This way, you can at least make sure the HT side of ignition system is in good working order, that is, the coil, rotor, cap and spark plug leads. The usual vendors offer a variable gap spark tester for about $10 but you can make one yourself just as easily out of an old spark plug for nothing.

Here's how:

Step 1

Take an old spark plug and cut off the L-shaped tip,
and clean up the center electrode and surrounding metal with a bit of sandpaper.

Step 2

Drill a small hole in the thickest part of the metal (where the wrench goes on). Use a metal screw to attach a length of wire, a couple of feet, to the body of the spark plug. To the other end of the wire, solder or otherwise attach a big alligator clip. (Witness my amazing Photoshop drawing skills)

Using the Spark Tester

Since the tip is cut off, the spark will have to be stronger to jump the wider gap. It is harder for a spark to form in a compressed cylinder so it more accurately represents the spark performance under running conditions. With the engine off, remove the spark plug wire you wish to test from the spark plug. Attach the spark tester to the plug wire and connect the alligator clip to a good ground on the car. Remember that the coil output can give you a very nasty shock (especially if you have a 40kV sport coil) so make sure to wear thick gloves or use a rag to hold the tester, and not to touch the body of the car when the system is energized. Hold the tester away from the car body and use your remote starter switch or have an assistant start the car (or simply turn it over). You should get a healthy bluish white spark from the center post to the side of the spark tester.