Ignition Failures and Solutions for Classic Motorcycles

In the quarter century that I have worked at dealerships and owned classic motorcycles one certain fact has emerged – ignition systems fail.

The “How and Why” are often a mystery, even to some mechanics. The goal of this article is to educate you about the function of an ignition, explain the cause of common failures, and review current solutions on the market today.

What parts make up the ignition of a motorcycle?

Three parts: Coil, ignition trigger, and spark plug.

Most motorcycle coils look like a can of soda with spark plug wires sticking out the end. The function is simply to convert your 12 volt battery power to a much higher voltage that can fire your spark plugs.

All ignition systems have a trigger of some kind, either a mechanical “points set” or magnetic sensor to turn the coil on and off at the correct time.

How does an ignition work?

On older engines the contact points open and close using a spinning lobe mounted to the cam or crankshaft. When the points CLOSE, power flows through the coil creating and storing high voltage energy.

It takes a certain amount of time (lobe rotation) to charge up the coil. The amount of rotation needed is called Dwell and is critical to proper coil operation.

Too little time=engine misfire from lack of coil energy

Too much time=excess heat and eventual coil failure

When the contact points open, it shuts the coil off causing the high voltage energy field to collapse, travel down the spark plug wire and make your plugs SPARK.

What usually fails?

When contact points or coils start to fail most riders believe the spark plugs fouled or the carburetors need adjusting. During normal use your mechanical points become dirty, poorly adjusted, and wear out from use. Coils can short out internally from age, vibration, or cracking of the coil body. Many times these failures cannot be detected without complicated and expensive equipment.

“Not all ignition failures cause your engine to stop running.”

Have you ever tried bending an old spark plug wire and noticed how stiff it is, or sometimes how gooey the insulation inside the wire has become? There is no way for the spark plug cap to securely thread onto that wire. You are better off replacing the wires but many motorcycle coils have the wires bonded into them. Poor electrical connections lead to ignition failures!

1) How to test for a bad coil?

Coils are made of tiny wires neatly wrapped around a metal beam thousands of times. Each layer is typically covered with thin paper to protect against short circuits. The protective coating on the wires can crack from age, vibration, or excess heat. Most service manuals give resistance test for coils. If a coil tests bad you are lucky. More often the coil starts to lose voltage causing difficult cold starting, rough idling, or loss of power and mileage. This type of failure doesn’t always show up when testing a coil. Replacing the defective coil is often the only guarantee that you’ve solved the problem.

2) How to tell if your points are bad?

You can clean and adjust the point gap, verify dwell is correct, and make sure the cover is secure so they stay dry. As you ride your motorcycle you will need to clean and adjust the points to keep your engine running smoothly. Modern engines have electronic ignition control for more reliable operation and improved performance.

Electronic ignition sensors use a magnet or set of magnets to trigger your coils. They are more accurate, never need to be cleaned or adjusted, and are relatively inexpensive. Modern cars and cycles use them and you can purchase upgrades for your classic cycle as well.

What options exist to replace or upgrade your ignition?

Factory replacement coils are seldom available. You can purchase used coils from a junk yard or internet seller. Most aftermarket coils use basic paper wrapped coils that still use 1960’s technology. For best results choose a replacement coil that uses Section Bobbin construction. They are much stronger and virtually eliminate internal short circuits. Use a “multi-sparking” coil for improved cold starts, smoother engine idle, and increased efficiency.

When upgrading your factory ignition, just about any electronic system will be an improvement. Most electronic ignitions use Hall Effect (magnetic) sensors to trigger the coils because they never need to be adjusted. For the best upgrade, use the new optical reader ignition which is far more accurate and is capable of superior timing and dwell control, and provides a more stable spark at all rpm ranges.