How Do I Know If My String Trimmer Clutch is Bad? (5 Signs)

The easiest way to tell if your weed eater’s clutch is either going bad or has gone bad is if you notice the clutch not reacting how it should. It’s that easy. If you’re pulling the throttle on your weed eater and something doesn’t feel quite right, then you’ll be right in thinking there could be an issue with the engine’s clutch. So, how do I know if my trimmer clutch is bad? Well, let’s go over the signs that would lead you to realize the clutch has an issue.

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Spotting a Bad String Trimmer Clutch (The Short Answer)

Inside your weed whacker, you’ll find a centrifugal clutch that engages and disengages in relation to the engine’s speed. If the clutch mechanism is faulty, then the process of engaging and disengaging is disrupted, making your weed whacker act differently. Here are five symptoms you might encounter with a bad clutch.

The Head Spinning at Idle
The Head Won’t Spin at High RPMs
The Head Takes Time to Engage at High RPMs
The Clutch Makes a Ringing Sound as it Engages
You Feel a Jolt When the Clutch Engages

How to Tell If Your String Trimmer Clutch is Bad (5 Symptoms)

Now that you know the five common symptoms that signal your weed whacker clutch is going bad, let’s take a look at each of them in more detail and figure out what’s actually going on.

The Head Spinning at Idle

One of the first symptoms you’ll probably notice is that your trimmer head spins at idle. This is caused by the clutch not being able to disengage. Inside the clutch is a spring or, on some particular models of weed whackers, a series of springs that pull the clutch shoes back into their disengage position.

Sometimes the spring comes loose or breaks, causing the clutch shoes to remain in the engaged position. So, even when your weed eater is ticking over, the head will continue to spin. Also, if the inside of the clutch gets wet, rust can build up and prevent the clutch shoes from pivoting. Usually, what happens is the clutch shoes get stuck in the open position and are no longer able to disengage.

The Head Won’t Spin at High RPMs

There are a few reasons your weed eater head won’t spin when you increase the throttle. The first is that the shoes of the clutch wear down over time. When this happens, the clutch slips since the clutch shoes and the clutch bell can’t make good contact. Also, as the shoes become really worn, they make little to no contact, preventing the head of your will eater from turning at all.

Finally, rust in the clutch mechanism can cause the clutch shoes to become stuck in the disengaged position, preventing them from engaging. Even though, in my experience, rust usually keeps the clutch engaged, it’s not uncommon for the clutch to get stuck in either position if the corrosion is bad enough.

The Head Takes Time to Engage at High RPMs

You may find that your weed eater head takes a few seconds to engage after you apply full throttle. Again this is one of the early signs that your clutch is on its way out. The clutch shoes need a sustained amount of centrifugal force to eventually open up and grip the clutch bell.

Typically, you will find that either the rust is causing the problem or there could be an issue with the spring. Either way, you’ll need to open up the clutch and figure out what is going on.

The Clutch Makes a Ringing or Grinding Sound as it Engages

One thing you might notice if your clutch is late to engage due to a bad clutch shoe or problematic spring is your weed eater making a grinding noise or a ringing sound. This is where the clutch shoe engages quickly and aggressively. The clutch shoes strike the clutch bell with force, which causes the clutch bell to make a grinding or ringing noise.

You Feel a Jolt When the Clutch Engages

A jolt is another symptom created when the clutch takes longer to engage. You’ll feel a jolt because the clutch is engaging with a sudden shock rather than a gradual engagement. So, you are feeling the clutch shoe gripping the clutch bell very quickly and transferring the force into the body of the weed eater.

Not only is this a sign of a bad string trimmer clutch, but it’s also an indication your weed eater is under immense stress. This stress affects not only the flexible drive shaft but also the engine.

What to Do If Your Trimmer Clutch is Bad

The first thing you should do if you notice your weed eater clutch not working is to switch off the weed eater. Clutch issues can lead to several different problems beyond just the clutch. Therefore, you’ll want to repair it as soon as you can.

Now that I have explained how you can tell if your trimmer clutch is bad, it’s time to look at the solution. Here are three solutions to repair a bad clutch depending on the findings of your diagnosis.

Cleaning Up a Corroded Clutch
Replacing a Damaged Clutch Spring
Replacing a Worn Out Clutch

Cleaning Up a Corroded Clutch

Nine times out of ten, a corroded clutch is actually fine and doesn’t need replacing. All you need to do is clean it up. More than likely, the corrosion is preventing the clutch from engaging and there isn’t any damage or wear. Also, it’s only the locations where the clutch shoes pivot that need your attention.

So let’s go over the steps to clean up your clutch. Here’s an example of a Stihl FS-50 clutch cleaning process.

Weed Eater Clutch Removal

Remove the screws securing the engine cover to the engine.
Lift off the engine cover.
Remove the screws securing the main weed eater shaft to the engine.
Pull the main shaft away from the engine to expose the clutch.
Remove the bolt securing the clutch shoes to the flywheel.
Lift off the metal plate covering the clutch shoes.
Lift the clutch mechanism off of the flywheel.

Weed Eater Clutch Cleaning

Disassemble the clutch shoe mechanism.
Place all the individual clutch shoe mechanism parts into a small container and soak them in white vinegar or a chemical rust remover for 24h.
Remove the clutch parts from the cleaning solution.
Clean the clutch parts using a wire brush and wire wool.
Give each of the parts a good coat of WD-40 and allow them to soak for 20 minutes.
Wipe the parts dry with a clean shop rag.

Weed Eater Clutch Assembly & Installation

Reassemble the clutch shoe mechanism.
Position the clutch mechanism back onto the weed eater’s flywheel.
Position the metal cover plate on top of the clutch.
Install the clutch shoe bolts.
Test that the shoes can open and close by hand.
Install the main shaft of the weed eater and install the screws.
Install the engine cover and screws.

Tools & Materials To Clean a Weed Whacker Clutch

Screwdriver
Torx Screwdriver
White Vinegar or Chemical Rust Remover
Shop Rags
WD-40
Small Container or Bucket

Replacing a Damaged Clutch Spring

I strongly recommend getting a small spring puller to replace a spring on a weed eater clutch. A spring puller is a small hook with a handle that makes installation quick and easy and reduces the risk of damaging your new spring. I have tried this repair with a pair of pliers and found myself both struggling and messing up my new spring. So, using the right tools is definitely worth the $2.

For this repair, I’ll use a Stihl FS-50 as an example.

Installing a New Weed Eater Clutch Spring

Remove the screws securing the engine cover to the engine.
Lift off the engine cover.
Remove the screws securing the main weed eater shaft to the engine.
Pull the main shaft away from the engine to expose the clutch.
Using your spring puller, remove the old spring from the clutch.
Hook one end of your new spring into one of the mounting points on one of the clutch shoes.
Using your spring puller, pull the spring and mount the spring into the second clutch shoe.
Test that the shoes can open and close by hand.
Install the main shaft of the weed eater and install the screws.
Install the engine cover and screws.

Tools & Materials To Clean a Weed Whacker Clutch

Screwdriver
Torx Screwdriver
Small Spring Puller
Replacement Clutch Spring

Replacing a Worn Out Clutch

Now it’s time for the last repair of installing a new clutch. Just so you know, a new clutch for a weed eater like a Stihl FS-50 costs about $20-30, so it’s not all that expensive.

So, if you don’t feel like cleaning the clutch or replacing a clutch spring, then you could always just opt for installing a new clutch entirely. Sure, it’s more expensive than $1 for a new spring, but it’s less of a hassle than getting new tools like a spring puller and sneaking vinegar out of the kitchen while nobody is looking. I’ll leave this decision up to you.

So, let’s take a look at the steps.

Steps to Install a New String Trimmer Clutch

Remove the screws securing the engine cover to the engine.
Lift off the engine cover.
Remove the screws securing the main weed eater shaft to the engine.
Pull the main shaft away from the engine to expose the clutch.
Remove the bolt securing the clutch shoes to the flywheel.
Lift off the metal plate covering the clutch shoes.
Lift the clutch mechanism off of the flywheel.
Place the new clutch onto the flywheel.
Position the metal cover plate on top of the clutch.
Install the clutch shoe bolts.
Test that the shoes can open and close by hand.
Install the main shaft of the weed eater and install the screws.
Install the engine cover and screws.

Tools & Parts to Install a New String Trimmer Clutch

Screwdriver
Torx Screwdriver
Replacement Clutch