When troubleshooting motor problems it’s handy to understand how the various systems work. Fortunately ignition is quite reliable and rarely causes problems beyond the spark plug. This may help when it does.

Basic Coil Description

A correctly timed spark is the system’s ultimate goal. It starts with a magnet, generating coil and adds some electronics which help generate the high voltage and time the events. Timing cues come from the magnet’s position on a flywheel or other disc.

Some systems provide an output for charging batteries, too. Usually they do this by placing additional generating 

coils around the flywheel. Generating coils are relatively low voltage but create a fair amount of current.

The diagram at right is a simplification since it does not show the electronics, usually buried inside the coil unit, to make it work. Two electrical concepts are used: 1) suddenly stopping current flow in a coil produces a high voltage in the coil and 2) running a changing current flow through a coil with few windings will induce a much larger voltage in a nearby coil with more windings.

Essentially, the guts do the following. All the electricity gathered from the generating coil gets put under great electrical pressure (voltage) then fired in a brief, well-timed pulse to the spark plug.

The Kill Switch shorts out the generating coil. That prevents the generating coil from sending anything to the rest of the system and is why the kill switch involves relatively low voltage. You won’t get much shock even if a kill wire gets exposed.

Common Problems for No Spark

1. The most likely cause of ignition problems is the spark plug. At $2 a copy replacing it is also the easiest, cheapest thing to do. Sometimes a motor can defy your most fervent corrective efforts when all it wanted was a new plug. Plugs can look surprisingly perfect–the right color, no visible cracks, shiny metal, etc and still be bad. If in doubt, toss it out.

2. Next up is the spark plug cap, namely those where the cap screws into the plug wire (most of them). It’s easy to mis-thread and not get firmly enmeshed in wire. It may work if its barely touching the wire but may cause intermittency problems. Those are the worst–a part-time problem.

3. The kill switch wire that goes from the coil out to your throttle button cannot touch any metal. If it does, it’s just like pressing the kill switch–the motor will not run.

4. If you have replaced any part of the motor requiring removal of the flywheel or magnet, then you must insure they’re returned to their proper home.

Thanks to Nick Scholtes for helping with technical details and research