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Harley Engine Knocking Sound: Should You Be Concerned?

Chase Manhattan Avatar

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Is the rhythmic tap-tap-tap of your Harley’s engine beginning to sound more like a worrisome knock-knock-knock?

As an experienced rider, it’s crucial to distinguish between the sweet symphony of a well-oiled machine and the death knell of an engine problem. Our article is designed to guide you through an auditory exploration of your motorcycle. Unearthing potential issues behind the dreaded Harley engine knocking sound, we bring forward key observations, precautions, and solutions underpinned by seasoned expertise.

Remember, the devil is in the detail (or decibel)! So, slip on your riding gloves, rev that engine, and let’s delve into those curious, concerning sounds echoing from your Harley’s heart. Because understanding your Harley’s language and responding in time can be the difference between a minor tune-up and a major overhaul. Let’s get to it.


You Ask, Why Is My Harley Davidson Engine Knocking?

A knocking sound from your Harley’s engine can be caused by several factors, most of which revolve around the engine’s internal combustion process. The most common cause is often premature detonation, which is the ignition of the fuel/air mixture before the spark plug fires. This problem can lead to a host of issues, including rough idling, poor acceleration, high operating temperatures, and the dreaded knocking sound.

Here's a cutaway image showing the internals of an engine. This is utilized to illustrate the delicate alignment of the engines internals, which can cause a knocking sound when out of alignment.
See the low margin for error when we’re dealing with delicate internal engine parts?

A Deeper Dive Into Harley Engine Knocking Sounds and Their Causes

Understanding the fundamental reasons behind the knocking sound in your Harley’s engine can help you take preventative measures and make timely repairs. Here are some of the common causes:

Using Low Octane Fuel

High-performance engines like those in Harley Davidson motorcycles require high-octane fuel for efficient combustion. Low-octane fuel takes longer to burn, which can lead to multiple detonations and cause a knocking sound.

This is an infographic detailing what happens when low octane fuel is used in an internal combustion engine that requires higher octane fuel.
Notice the ignition points in the knocking engine – this is what’s known as preignition or spontaneous combustion, in each case respectively.

Fouled or Worn Spark Plugs

Spark plugs that are worn out or fouled can lead to the fuel/air mixture in the engine cylinders igniting at the wrong time. This incorrect timing can cause a knocking sound.

Engine Is Running Lean

A lean-running engine, one in which there isn’t enough fuel for the amount of air in the cylinders, can cause incomplete combustion. This results in unburnt fuel molecules that create a knocking sound.

Engine Is Overheating (Not Cooling)

If the engine’s cooling system is not functioning properly, the temperature inside the cylinders can rise significantly. This can lead to premature ignition of the fuel and cause a knocking sound.

Compression Ratio Is Too High (Rare)

A high compression ratio can cause the air-fuel mixture in the cylinders to ignite prematurely. Although this is a rare occurrence in Harley motorcycles, it can cause engine knock.

Related: Harley Bad Compensator Symptoms Get Revealed


Internal Engine Damage Potentials

While the causes mentioned above are common, some more serious internal engine problems could also lead to a knocking sound:

This infographic is highlighting the cylinder walls and the piston inside of them. This introduces them as the two main causes of engine knocking.
Here’s a closer look at the cylinder walls and their relation to the piston, and how this delicate balance can be interrupted to cause a knocking sound.

Cracked Cylinder Walls

Cracks or warping in the cylinder walls can cause the air-fuel mixture to escape before it’s fully combusted. This premature ignition can cause a knocking sound.

Piston Slap

Piston slap, a condition where the pistons aren’t properly fitted or are excessively worn, can also cause a knocking sound. This happens due to an increased clearance between the cylinder walls and the piston rings.

What Is Piston Slap

Piston slap is a knocking noise that’s heard when the bike’s engine is first started and is particularly noticeable in colder weather. It’s caused by the piston moving sideways in the cylinder bore and ‘slapping’ the cylinder wall.

Why Does It Happen?

The cause of piston slap is usually due to wear and tear over time. This wear can lead to an increased clearance between the cylinder walls and the piston rings, resulting in an audible knocking sound.

Related: Harley Crank Runout Symptoms: Identifying and Addressing the Issue


How Do You Know When To Be Concerned About These Issues?

While a certain level of noise from your engine is normal during operation, a knocking sound is not. This sound, especially if it’s loud and persistent, could indicate a serious issue with your engine and should be checked out as soon as possible.

This video isn’t motorcycle specific, but I found it highly informative for the topic we’re covering today. I recommend checking it out to really understand the concepts behind the knocking sound that you may be experiencing. Video titled “A DETAILED overview of KNOCK and PRE-IGNITION – BOOST SCHOOL #7”

My Advice When You Notice A Knocking Sound

If you notice a knocking sound from your Harley’s engine, don’t ignore it. Seek professional help from a trusted mechanic or your nearest Harley service center. Regular maintenance and using the right oil and fuel can also go a long way in preventing engine knocking.

Related: Harley Oil Sumping Fix: A Comprehensive Repair Guide

Wrapping Up

A knocking sound in a Harley Davidson engine is not something to take lightly. It could be a sign of a serious problem that, if left unaddressed, could lead to severe engine damage and expensive repairs. If you hear a knocking sound, seek professional help immediately. Regular maintenance, using the right fuel, and promptly addressing any issues can help keep your Harley’s engine running smoothly for a long time.

Remember, prevention is always better than cure. Take good care of your Harley, and it will take good care of you.


What Causes a Knocking Sound in a Harley Engine?

A knocking sound in a Harley engine can be caused by various issues, including detonation, improper fuel octane, worn engine components (like bearings or pistons), or inadequate lubrication. Diagnosing the exact cause is crucial to prevent potential engine damage.

Is It Safe to Ride My Harley with an Engine Knocking Sound?

Riding a Harley with an engine knocking sound is not recommended. This sound could indicate a serious issue that, if left unaddressed, could lead to engine failure or other safety hazards. It’s best to have the motorcycle inspected by a professional.

What Should I Do If My Harley’s Engine Starts Knocking During a Ride?

If your Harley’s engine starts knocking during a ride, it’s advisable to stop riding as soon as it’s safe to do so. Continuing to ride with a knocking engine can cause further damage. After stopping, check for obvious issues and consult a mechanic.

Can Engine Knocking in a Harley Be a Sign of a Serious Problem?

Yes, engine knocking in a Harley can be a sign of a serious problem. It often indicates issues like engine detonation, wear and tear of critical components, or lubrication problems, all of which require immediate attention to prevent major engine damage.