There are countless reasons why your Harley won’t start, has rough idling or even stalls. The battery might be reaching its end or is completely flat, the carburettors might be clogged, or you might be running it on bad fuel. The

There are countless reasons why your Harley-Davidson motorcycle won’t start, is idling rough, or stalls out. Causes can range from a battery reaching its end or being completely flat, the bike’s carburetors might be clogged, or having lousy fuel in its system. Then, there may also be issues with the internals, like off timing or the engine failing to tick over since it’s seized. Finally, blame poor maintenance and, more commonly, lack of proper lubrication. 

But how many of us have considered that it might be down to problems with the ignition? After all, starters can fail, spark plugs get dirty or foul up, electric writes corrode, or the ignition coil may be acting up. These mechanical issues are more common than you think but can usually be resolved quickly and cheaply.  

Motorcycle Ignition Systems 

The purpose of the ignition system in motorcycles, and any vehicle with an internal combustion engine, for that matter, is to deliver enough current to the spark plugs to create a spark. The spark ignites the air and fuel mixture, pushing the cylinder head downward. The whole process needs to occur precisely to create proper combustion and power delivered to the back wheel. 

As a piston nears the Top Dead Center (TDC) on the uptake, the spark plugs are readying to do their thing. All of this is within milliseconds and happens thousands of times each minute. Timing that is slightly off, or spark plugs with faulty voltage, will cause your engine to cough up unburnt fuel through the exhaust and lose power.  

If the other electrical system components are okay, including the battery, switchgear, and parts responsible for charging, then the issue lies with the ignition coil. Older bikes use a different kind of ignition coil. For example, a “points” coil for Harley Davidson motorcycle generally uses two mechanical points that open and close the circuit as they spin around a shaft tasked to move them. Here engine speed plays a part. The faster the engine turns, the more voltage is delivered to the plugs. Unfortunately, points ignition systems had one major downfall – advanced wear, which meant more issues and more maintenance.  

Harley Davidson started introducing electronic ignition in its bikes in the early 1980s. This differs considerably from mechanical systems. Reliability is improved, especially in parts wear and timing, hence better overall consumption and emissions. There is also room for increased performance with more current going to the spark plugs, creating a bigger bang. The critical role here is that of the ignition coil.

n, there may also be issues with the internals, like off timing, or even the engine failing to tick over since it’s seized. Blame poor maintenance and more commonly lack of proper lubrication. 

But how many of us have considered that all might be down to problems with the ignition? The starters may fail, the spark plugs can get musty, the wiring may be damaged, or simply the ignition coil may be acting up. These issues are more common than you think. Luckily, they can usually be resolved quickly and cheaply.

How Ignition Coils Work 

Ignition coils are part of the motorcycle’s ignition system. Their job is to increase the voltage coming from the battery. Twelve volts won’t get you anywhere, so the coils increase the battery’s input voltage to a proximity 30000-Volts. 

Essentially, a Harley Davidson motorcycle coil is a step-up transformer. It consists of two separate wire coils, called a ‘primary’ and ‘secondary’ coil, each wound around a central iron core. The primary coil has several windings of thicker wire, whereas the secondary coil has thousands of windings in thinner gauge wires. When a spark is needed, the 12V from the battery is cut off. As a result, it causes a drop in the magnetic field in the primary coil but induces a much higher voltage in the secondary coil necessary to work the plugs.  

As with everything, timing is crucial here. The right amount of voltage at the right time is controlled by the onboard computer that receives input from a series of sensors. These sensors respond to changes in the magnetic fields in the coils. Computers sync the work of the injectors, so the right amount of fuel is ignited at the correct temperatures and at the right time.  

Types of Coils Found on Harleys 

Most Harley Davidsons are V-twins, so there are two types of coils found on Harley bikes – dual fire and single fire. Dual fire coils provide stepped-up voltage to both cylinders and spark plugs simultaneously, no matter if one cylinder is in the compression and the other cylinder in the exhaust stroke. Single coils send current only to the cylinder in the compression stroke. This should provide smoother engine performance, particularly when idling.

However, there won’t be evident performance gains in using either type. Though there is a price difference – a dual coil setup is often cheaper. 

A recent variation of single coils is the Coil on Plug (COP) units. These have coils positioned directly on top of each spark plug, doing away with the spark plug wires and distributors. The result is that there is no inherent resistance and voltage loss from the wires, so the voltage is consistent. In addition, electronics improve the timing during the combustion stroke, so less fuel goes to waste. 

When to Replace Your Existing Coils 

In general, stock coils should last throughout the lifetime of the motor. But you might be out of luck and have a defective coil after a couple of thousand kilometers showing on the odometer. I mentioned a few of the typical signs in the bike when the coils play up, but let’s get into some detail. 

Backfiring – an engine that backfires has either a timing issue, not enough spark for the fuel/air mix, or an incorrect air to fuel ratio. As a result, the exhaust emits a popping sound and expels white smoke, followed by a loss of power. Coils that cannot produce enough current to create a spark for optimal combustion are one place to troubleshoot if your Harley-Davidson motorcycle begins to backfire.

Sudden loss of power and stalling – extremely low voltages signify that the coil is defective. This will result in a gradual or sudden loss of power that can cause the motorcycle to stall. In worse-case scenarios, you won’t be able to start the bike again.  

Poor fuel mileage – higher fuel consumption over the same terrain and your usual riding speed is a worrying sign. Dozens of things can cause it but worn spark plugs and faulty coils are some of the main factors. Over time, issues related to fueling and timing can arise.  

Rough Idling – Harleys are not known for the smoothest idling due to the engine design, but the engine shouldn’t be choking or jolting when idling. Defective coils have a hand in this one. If the engine stalls at idle, then certainly check the condition of the coils.  

There are three paths to fixing coil issues in Harley-Davidson motorcycles. The first path is hiring a specialized Harley-Davidson dealer or certified technician to replace the ignition coil. If you’re mechanically apt, the second option is to try and fix/replace the coil yourself. Lastly is the replacement of the entire ignition control module.  

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Cover Photo credit: Austin Neill (@arstyy) | Unsplash Photo Community