Weed Eater Making Grinding Noise (I Figured Out Why)

A weed eater making a grinding noise is never a good sign. It’s an unwelcome sound signaling that your weed eater has a problem. But where is it coming from, and is something broken? Well, there are a number of different issues that could be causing the noise. So, let’s take a look at the potential problems and figure out what can be done.

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Weed Eater Making Grinding Noise (The Short Explanation)

A grinding noise coming from your weed whacker is likely caused by a fault with the centrifugal clutch, flexible drive shaft, or gear head mechanism. Also, the weed eater will only make the grinding noise while the weed eater head is in operation. The grinding noise indicates that your weed eater needs servicing before any further damage is caused.

4 Possible Causes for a Weed Eater That Makes a Grinding Noise When in Use

You will probably find that your weed eater only makes a grinding sound when you operate the head, which tells you that it’s probably not caused by an issue with the motor. Therefore, one of the moving parts after the engine is causing the grinding sound. Here are four possible reasons why your weed eater is making a grinding noise.

Centrifugal Clutch

You’ll find a centrifugal clutch between the motor and the flexible driveshaft. The clutch is the device that stops and starts the head rotation as you engage/disengage the throttle. Without this clutch, your weed whacker head would spin continuously.

If something breaks inside the clutch, you will likely hear a grinding sound as the metal parts rub against each other. So, use a screwdriver to remove the weed eater’s engine cover, then remove the main shaft from the engine. When you remove the main shaft, you’ll expose the inside of the clutch.

If the grinding is coming from the clutch, then you should spot the damage easily, as some of the metal will probably be shiny where they have been rubbing, or there’ll be an obvious break in one of the parts.

Flexible Driveshaft

Your weed eater uses a flexible drive shaft to transfer the motor’s power to the head. The driver shaft is flexible so that it can bend through the bent shaft and also so that it can flex as torque is applied. If the driveshaft were rigid, the torque from the engine would very quickly twist and snap the drive shaft.

In my experience, if you put your weed eater under a lot of stress, say, knocking down big weeds or trimming a lot of thick grass, then the flexible drive shaft can snap. If this happens, the broken ends of the drive shaft will catch one another and possibly still turn the head. So, the break can rub on the inside of the shaft, or the two ends can spin against each other.

Also, you could find that with the force of the torque from the weed eater engine, the driveshaft could have started to twist on itself, resulting in the driveshaft rubbing on the side of the main shaft. Either way, you’ll probably hear some kind of grinding noise.

To check the condition of the drive shaft, you’ll first need to remove the main shaft from the weed eater to expose the clutch. Then pull the clutch bell from the main shaft.

You’ll also pull the flexible driveshaft when you pull the clutch bell. So, if you find that the driveshaft is either snapped or twisted, this is the cause of the weed eater making a grinding noise.

Gear Head Mechanism

Depending on the type of weed eater you have, you might find that the line head is attached to a gear mechanism. This is a device that allows the head to be at a different angle from the main shaft. If you have a weed eater with a curved shaft, then you likely don’t have a gearhead, as the curve of the shaft makes this change in angle. Again, this is an example of why the driveshaft is flexible.

If you have a straight-shaft Stihl, Echo, or even Husquvarna string trimmer, you more than likely have one of these gear heads. So, what has gone wrong? Well, like any gear, there is a certain amount of wear and tear that will, in time, cause the gears to fail. But more often than not, they fail due to a lack of lubrication.

Lubrication, what lubrication? You might be asking. Well, if you closely inspect a serviceable gear head, you’ll find a small removable screw bolt. This is where you should be adding grease on a regular basis.

So, how do you know if your weed eater making a grinding noise is due to the gear head? Well, all you need to do is remove the gear head from your weed eater and give it a spin by hand. Then, you’ll be able to tell pretty quickly if the gears are shot or not.

Debris in the Gear Head

The last issue I want to mention as a potential reason your weed eater makes noise is debris between the gear head and the line head. If you take a close look at these two parts of your weed eater, you’ll see that there is a small gap between the two parts. This gap is the perfect size for tiny rocks to fit between. So the grinding noise could be as simple as metal rubbing against a few small rocks.

How to Fix a Noisy Weed Eater (4 Things to Try)

Now it’s time to take a look at how to repair each of these four issues. Here are all the steps you’ll need to follow to repair your weed whacker, including all the tools, materials, and parts.

Replacing a Centrifugal Clutch

When replacing the clutch on your weed eater, there are two parts you might need to replace: the clutch shoe mechanism and the clutch bell. So, I have broken this repair down into two sections for each part. Here is an example of how to replace the clutch parts for a Stihl-FS-55, which is pretty much the same for most gas-powered weed eaters.

Replacing the Clutch Shoe Mechanism

Remove the screws securing the engine cover to the motor.
Remove the spark plug cable.
Remove the engine cover.
Remove the main shaft bracket bolts.
Pull the main shaft from the weed eater.
Remove the two bolts securing the clutch to the flywheel.
Lift the old clutch from the flywheel.
Place the new clutch onto the flywheel, making sure that the arrow on the clutch is. pointing in the correct direction.
Reinstall the two clutch securing bolts.

Replacing the Clutch Bell

With the engine cover already removed, remove the flexible drive shaft from the clutch bell.
Place a Torx screwdriver into the end of the clutch bell and remove the screw.
Lift the clutch bell off of the driveshaft attached to the flywheel.
Position the new clutch bell onto the drive shaft.
Install and tighten the Torx screw into the drive shaft.
Finally, reinstall the main shaft, engine cover, and all screws, followed by connecting the spark plug.

Tools & Parts to Replace a Weed Eater Clutch

Replacement Clutch Bell
Replacement Clutch Shoe Kit
Torx Screwdriver

Swapping Out a Flexible Driveshaft

The process of changing the flexible drive shaft varies between weed eater models and designs. The main difference is some models have a clutch bell fixed to the flexible driver shaft, and some don’t. What this means is that the models without a connected clutch bell to the driveshaft don’t require you to remove the engine cover and the clutch bell first.

For example, the Stihl-FS-45, a common homeowner weed eater, doesn’t require any disassembly of the engine or the clutch. So, let’s go over the steps for this weed eater design.

Steps to Replace a Flexible Driveshaft

Loosen the screws on the weed eater handle, leaving them in place.
Remove the single screw passing through the weed eater case into the main shaft.
Pull the main shaft from the engine.
Remove the small screw from the side of the main shaft located beside the line head.
Pull the bearing house from the main shaft, including the flexible driveshaft.
Detach the bearing housing from the flexible driveshaft.
Thread the new flexible driveshaft into the main shaft making sure to orientate the new driveshaft correctly. Top at the top and bottom at the bottom.
Connect the flexible driveshaft to the bearing housing.
Slide the bearing housing with the flexible drive shaft attached back into the main shaft.
Install the small screw into the side of the main shaft.
Reinstall the line head.
Slide the main shaft back into the engine case.
Turn the line head until the flexible drive shaft lines up with the clutch bell inside the clutch.
Once both the flexible drive shaft and clutch bell are aligned, slide the main shaft back into the engine case.
Reinstall the main screw into the top of the driveshaft to lock the main shaft in place.
Tighten the remaining screw on the weed eater case.

Tools & Parts to Replace a Weed Eater Flexible Drive Shaft

Replacement Flexible Drive Shaft
Torx Screwdriver
Pliers

Installing a New Weed Eater Gear Head

Again, weed eater gear heads come in all shapes and sizes, so no two repairs are exactly the same. But they all seem to detach and attach in pretty much the same way. So, let’s use the Stihl-FS-94 as an example and go over the steps.

How to Replace a Weed Eater’s Gear Head

Remove the clipping guard from the end of the main shaft.
Remove the line head from the weed eater.
Remove the screw securing the gear head to the main shaft.
Pull the gear head from the main shaft.
Slide the new gear head part way onto the main shaft.
Slowly turn the gear head until the flexible driveshaft and the opening at the end of the gear head are aligned.
Fully push the gear head onto the main shaft.
Secure the gear head to the main shaft.
Install the line head.
Install the clipping guard.
Remove the screw bolt to access the grease insert point.
Screw the grease tube into the opening.
Add grease to the gear head.
Remove the grease tube and reinstall the screw bolt.

Tools, Parts & Materials to Replace a Weed Whacker Gear Head

Screwdriver (Torx for Stihl Models)
New Gear Head
Gear Head Grease
Socket Wrench Set

Cleaning Out Debris Between the Gear Head & Line Head

Finally, the last repair is to remove the junk from between the gear head and the line head. You’ll be glad to know that this repair works for pretty much all weed eaters, regardless of the make or model. This example is for a Stihl-FS-56.

Steps to Remove Debris from a String Trimmer Head

Place a screwdriver through the back of the gear head and into the line head.
Turn the line head until the holes in the line head and the gear head align.
Push the screwdriver through into the line head.
Twist off the line head ( Stihl Models Clockwise to Remove).
Pull the line head off of your weed eater.
Clean out the debris.
Reinstall the line head.
Remove the screwdriver.

Tools to Remove Debris from a Weed Eater Head

Screwdriver