10 Harley 120ST Problems (With Solutions)

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The Harley 120ST is a powerful motorcycle engine made by Harley-Davidson, a well-known American motorcycle manufacturer. 

Harley 120ST Problems
120 Cubic Inches of Twin Cam Muscle | SE Twin Cam 120 ST

While it is admired for its strong performance, some owners have reported various problems with this engine. 

In this article, we will focus on the most common issues, and provide you with different solutions to resolve these problems.

Let’s begin!

Harley 120st Problems

Harley 120st most common problems are engine overheating, excessive vibration, oil leaks, transmission issues, irregular idle, difficulty starting in cold, and electrical faults.

1. Engine Overheating

Engine overheating is one common problem that can occur with a Harley 120ST. The engine gets too hot, much like when you have a fever. 

This can happen due to several reasons; such as not enough cooling liquid/coolant which is flowing around the engine to absorb the heat it generates. 

Another reason could be that the radiator is not working properly. If you are facing an engine overheating issue, you should address it as soon as possible. 

One simple solution is to ensure the coolant level is adequate and the radiator is clean and in good condition. 

2. Excessive Vibration

The 120ST can sometimes have an issue known as excessive vibration. This problem means the motorcycle shakes a lot more than it should, making for an uncomfortable and unsafe ride. 

The vibrations could be due to unbalanced tires, incorrectly adjusted suspension, or problems with the engine mount. 

To resolve this, you may need to get the tires balanced, adjust the suspension, or have a professional check the engine mount. 

3. Oil Leaks

Oil leaks are another common issue reported by the 120ST owners. So, in this problem, oil keeps running out of the motorcycle, which is crucial for keeping the engine running smoothly. 

It’s like having a water bottle with a small hole – slowly but surely, water will start to dribble out. 

If this happens, the engine can’t work properly and may get damaged. The leak might be due to worn-out parts or a bad seal. 

To solve this, you should carefully inspect the areas where oil is leaking. If you find any sign of wear or tear you should replace that part.

However, if you are not comfortable doing this yourself, you should get it checked by a professional.

4. Transmission Issues

The transmission is like the bicycle’s gear system; it manages the power and speed of your motorcycle. 

When it has a problem, you might notice that the bike is hard to shift, or it might make unusual noises. 

This can be due to worn-out parts, lack of lubrication, or incorrect adjustments. 

So, you can solve this by regular maintenance, using appropriate transmission fluid, and promptly addressing any unusual sounds or changes in performance. 

5. Fuel System Problems

This problem might be due to parts within the system not functioning correctly. The fuel system is crucial in providing the right amount of fuel to the motorcycle’s engine. 

If it’s not working properly, the engine might receive too much or too little fuel.

This can make the bike harder to start, cause it to run roughly, or even lead to it stalling in the middle of a ride.

To resolve this, you’ll need to clean or replace the malfunctioning parts within the fuel system. 

So, you should check the fuel system regularly for any signs of wear, damage, or blockage to keep it running smoothly.

6. Electrical Faults

Sometimes owners encounter electrical faults, which means there’s a problem with the motorcycle’s electrical system. 

This could be due to a variety of reasons such as worn-out wires, faulty fuses, or a bad battery. 

Worn-out wires may interrupt the flow of electricity, resulting in poor performance or even preventing the motorcycle from starting. 

On the other hand, faulty fuses may cause specific electrical features to stop functioning properly. 

Lastly, a bad battery might not hold a charge, which leads to problems when you are trying to start the motorcycle. 

To fix these problems, you should replace the faulty parts, such as the wires, fuses, or the battery.

7. Difficulty in Cold Starts

This issue can be caused by several factors, such as a weak battery, fuel delivery problems, or spark plug issues. 

The battery might not provide enough power to turn the engine over, the fuel might not be reaching the engine properly, or the spark plugs might not be firing correctly. 

One possible solution could be to ensure regular maintenance of your Harley 120ST. 

This includes checking the battery’s condition, examining the fuel system, and replacing the spark plugs if needed.

8. Irregular Idle

This problem refers to the inconsistent performance of your motorcycle when it’s not moving, but the engine is running. 

In other words, when your motorcycle is sitting idle, instead of running smoothly, it might rev up and down on its own. 

This can be due to several factors such as a dirty air filter, poor fuel quality, or issues with the fuel injection system.

For a quick fix, you should clean or replace the air filter and ensure that high-quality fuel is being used. 

If the problem persists, it might be related to issues like the fuel injection system, in this case, you’ll need to get in touch with a professional.

9. Clutch Problems

Some riders have also reported problems with the clutch system, which is a device that connects and disconnects the engine to the wheels. 

If the clutch is not working properly, it can make it hard to change gears or it may cause the bike to jerk or stall. 

This problem can happen if the clutch plates are worn out or if the clutch cable is too tight or too loose. 

So, the solution to this problem is often to replace the worn-out clutch plates or adjust the clutch cable to the correct tension.

10. Crank Bearings Issues

Crank bearings in the 120ST engines can sometimes become weak or damaged. This could lead to poor performance of your Harley, even causing it to break down.

When that happens, the engine parts they support can’t move as they should. You might hear unusual noises, or the motorcycle might not run smoothly. 

The good news is, you can resolve this problem by replacing the damaged bearings with new ones.

You can also check out the Harley 96 engine103 engine107 engine110 engine114 engine117 engine131 engine and Evolution engine Problems.

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