Harley Engine Knocking Sound – What May Be Wrong?

The Harley Davidson engine is a complex machine with many moving parts. When one of these parts starts to fail, it can cause knocking sounds. These sounds can be an indication of serious problems, and should not be ignored. Putting off diagnosis and repair can lead to further damage and expensive repairs.

In this article, we will discuss the main reasons for the occurrence of a knocking sound in the engine of a Harley bike so that you could take care of the issue before it even gets a chance to get any worse.

Primary Cause of the Knocking Sound in the Harley Engine

According to experts, mechanics, and Harley owners familiar with the problem, the knocking sound in the engine of the bike is mainly caused by premature detonation. This is most likely the case if you're positive that the mountings of the engine plus other parts of your motorcycle are okay.

Premature detonation occurs when the fuel/air mixture is too lean, causing it to ignite before the spark plug fires. Premature detonation can be caused by many things like a clogged air filter or an incorrect carburetor setting, but it is most often due to low-octane gasoline.

Symptoms of premature detonation in Harley Davidson engines can include rough or irregular idle, poor acceleration and performance, higher-than-normal operating temperatures, disconnecting the spark plug wires while the engine is running, and a loud knocking sound from the engine.

This issue can cause serious damage to an engine if left untreated for too long. It can lead to piston ring failure, spark plug fouling, and even engine seizure.

It is important to take necessary precautions in order to prevent premature detonation in a Harley Davidson engine. Regular maintenance should be done on the air filter, spark plugs, and carburetor settings in order to make sure all components are running as intended. Additionally, it is also important to use only high-octane, unleaded fuel to reduce the risk of partial combustion.

If symptoms of premature detonation become apparent, it is important to diagnose and fix the issue as soon as possible to avoid any damage or further issues with the engine.

Factors Causing the Premature Detonation Leading to the Engine Knock in Harley Bikes

Here's a rundown of the most common factors that may trigger the engine knock in your Harley bike:

1. Use of Low Octane Fuel

Compared to high-octane gasoline that burns quickly, low-octane gasoline takes longer to combust when applied to high-performance engines with high-compression ratios. As a result, it has the tendency to detonate a couple of times more than normal.

The solution to this problem is to use high-octane fuel, which burns quicker and detonates only once. When combined with the correct air/fuel mixture and spark timing, high-octane fuel will help ensure the engine runs smoothly and efficiently.

Additionally, some Harley Davidson models may benefit from the use of a fuel additive to reduce the risk of detonation. This additive can help offset any variations in fuel quality that may affect the performance of the engine.

2. Faulty Spark Plugs

A faulty spark plug can cause premature detonation in the engine of a Harley bike. This happens when it is worn and no longer firing at the precise moment. Due to this, the timing inside the cylinders of the engine will be out of calibration, which will ultimately manifest as a knocking sound. To solve this problem, the spark plugs must be replaced with new ones of the same amperage.

On the other hand, if carbon build-up is the only thing blocking the proper connection of the spark plug, you can use a clean piece of cloth to remove the contaminant. For harder-to-remove grime, you can spray a spark plug cleaning solution to help loosen it up before carefully wiping it off with a wire brush. A sturdy knife can also be used to scrape off hardened deposits but be careful not to damage the sensitive components of your spark plug while doing so.

3. Engine Running Lean

When the engine runs lean, it means that not enough fuel is being supplied for the amount of air going into the cylinders. This causes an incomplete combustion process in which some of the fuel molecules remain unburnt and eventually turn into a knocking sound inside the piston chambers.

In addition to using high-octane fuel, you can bring back the right air/fuel ratio in your Harley engine by using a fuel injector cleaner and/or a fuel additive recommended by your vehicle manufacturer. This will help ensure the purity of your fuel and remove any carbon deposits that can block its flow into the cylinders.

It is important to check your air filter and replace it if dirty or clogged, too. A blocked filter means less air goes into the engine and causes the fuel to be in limited supply.  Once all of these components are running properly, it should help reduce the risk of premature detonation and ensure your Harley Davidson engine runs smoothly.

4. Inefficient Cooling System

If there is a problem with the cooling system in your Harley bike, it may lead to premature detonation and cause an engine knock. This happens when the temperature inside the cylinders gets too high, leading to the ignition of fuel before its intended time.

The solution to this issue is to ensure that your cooling system is properly maintained and checked regularly for any blockage. You should also make sure the coolant is topped up and check if the radiator fan is still spinning. Additionally, you can replace any damaged or faulty components in the cooling system such as the thermostat, water pump, hoses, and radiator to prevent the engine from overheating.

Finally, if you are looking for a more permanent solution, you can install a better-quality radiator plus a fan that offers more cooling capacity. This will ensure your Harley engine runs cooler and reduce the risk of premature detonation significantly.

5. High Compression Ratio

Having a very high compression ratio can also cause engine knocks in vehicles. This happens when the air-fuel mixture inside the cylinders is compressed more than necessary, leading to its premature ignition before it has time to be thoroughly mixed and burned.

Although the compression ratios of various Harley motorcycles are considered to be within the ideal numbers, you can opt to reduce it if you suspect that it is the one causing your engine knock or premature detonation.

So, one way to prevent this from happening is by installing a thicker gasket between the cylinder head and the engine block can regulate the compression ratio and keep it within a safe range.

Another way to reduce engine knock is by using high-octane fuel, as mentioned earlier, because this will provide more oxygen molecules in the air-fuel mix and prevent any detonation from occurring too soon.

Using ignition retarders can also help reduce engine knocks, as this will postpone the spark timing and give the air-fuel mix more time to burn. You can also install higher-quality spark plugs that are designed to withstand high compression ratios and prevent premature detonation from occurring.

6. Cracked Cylinder

If your Harley bike's cylinders are cracked, bored, or warped due to wear and tear over time, it can also result in premature detonation and engine knock. This is because the air-fuel mixture will escape through any crevices or cracks in the cylinder walls before it has time to be thoroughly burned.

The only real solution to this issue is to replace the damaged cylinder or have it repaired by a professional if it's still possible. You can also opt for a complete engine rebuild if that's necessary. In any case, replacing the faulty component will ensure your Harley Davidson runs like new and prevent any further engine knock from occurring due to premature detonation.

7. Piston Slap

Piston slap is another common cause of engine knock, especially in older Harley Davidson models. This happens when the pistons are not properly fitted or have become excessively worn over time, leading to an increased clearance between the cylinder walls and the piston rings. As a result, there will be an audible knocking sound from within the engine as the pistons move up and down in the cylinder.

The best solution to this issue is to have a mechanic check for any wear or damage on the pistons and replace them if needed. Furthermore, you may need to install new piston rings to ensure that there is an adequate seal between the ring and the cylinder walls. This will reduce the knocking sound significantly, allowing your Harley engine to run more smoothly and reduce the risk of premature detonation.

Conclusion

A knocking sound in a Harley Davidson engine should not be ignored because it could be a sign of a serious problem. The diagnosis should not be put off for long because the sooner the problem is found and fixed, the less likely it is to cause serious damage and expensive repairs.

If you suspect any of the abovementioned problems to be the cause of the engine knock in your Harley, don't hesitate to seek professional help from your trusted mechanic or your nearest Harley service center as soon as possible.