Gas Weed Eater Won’t Start? Try This. STIHL weed eater fuel

Gas Weed Eater Won’t Start? Try This

Weed eater, weed whacker, string trimmer – no matter what you call it, here’s how to get it running again.

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No matter what you call it – weed eater, weed whacker, string trimmer – chances are at some point it won’t start. Few things are more annoying than destroying your shoulder trying to start a gas weed eater when there’s work to do.

Fortunately, gas weedeater engines are pretty simple, so most DIYers with a few tools and some basic know-how can troubleshoot a stubborn trimmer and get it running.

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) Check The Gasoline

Gasoline can break down in as little as 30 days, especially today’s ethanol-containing gas.

Homeowners sometimes stash their string trimmer in the garage at season’s end without stabilizing the gas. Oxygen has all winter to break down and ruin the gasoline, leaving you with a trimmer that won’t start in the spring.

If your trimmer falls into this category, empty the old gas from the fuel tank and replace it with fresh fuel.

If your weed eater won’t start, trying removing the air filter and spraying carburetor cleaner into the intake.

) Clean The Carburetor

Once gas breaks down, varnish, gums and other debris can form inside the carburetor and clog the tiny fuel passages. This prevents fuel from reaching the combustion chamber and igniting, leaving you to struggle with a trimmer that won’t start.

Remove the air filter and spray carburetor cleaner into the intake. Let it sit for several minutes to help loosen and dissolve varnish. Replace the filter and try starting the trimmer.

If this doesn’t solve the problem, consider disassembling the carburetor to give it a more thorough cleaning.

Beware, however – taking apart a carburetor marks a point-of-no-return, of sorts. Understanding how the delicate gaskets, tiny screws and needle valves go back together can be a challenge, even on a relatively simple string-trimmer carburetor. Take pictures with your phone throughout the process to help reassembly. Clean all the openings and passages with carburetor cleaner.

If you’re reluctant to take apart the carb, visit the servicing dealer.

Remove the spark plug and use light sandpaper to clean electrode deposits to help fix a gas trimmer that won’t start.

Free Stihl Trimmer thrown away because it won’t start

) Clean/Replace Spark Plug

Oil deposits and carbon can foul the spark plug in a two-stroke engine if a low-quality oil is used. Deposits on the electrode prevent the plug from firing properly, which can reduce performance or prevent the engine from running altogether.

Plugs are inexpensive, so replace it if it’s fouled. If you don’t have a new plug available, clean the deposits from the electrode with light-duty sandpaper and check the gap. Consult the owner’s manual for the correct gap size.

If you know the spark plug is good, but the engine still doesn’t produce spark, the coil is likely to blame and requires replacement.

My weedeater has spark and fuel but it still won’t start

Direct compressed air from the inside of the air filter toward the outside to remove debris that may be restricting airflow.

) Clean/Replace Air Filter

A clogged air filter prevents the engine from receiving sufficient air to operate properly.

Before removing the air filter, brush away loose debris from around the filter cover and filter element. Tap rigid filters on a tabletop or the palm of your hand to dislodge any dirt or debris. Compressed air also works well. Make sure you direct air through the filter from the inside to avoid lodging debris deeper in the filter.

Avoid washing paper filters as this can collapse their micro-fine structure. Foam filters, however, can easily be washed using mild detergent and warm water.

As with the spark plug, however, replacement is often the best practice, especially if the filter is excessively dirty.

A spark-arrestor screen clogged with deposits can choke off airflow enough to prevent the trimmer from starting.

) Clean The Spark-Arrestor Screen

On many trimmers, a small screen covers the exhaust outlet and prevents sparks from exiting the muffler and potentially starting a fire.

As with plug fouling, too much oil in the gasoline, inferior oil and continued low-rpm operation can plug the screen with carbon deposits. This prevents exhaust-gas flow, which leads to power loss. In extreme cases, heavy deposits choke airflow enough to leave you with a weed eater that won’t start.

To fix the problem, remove the spark-arrestor screen and spray it with a heavy-duty cleaner, like AMSOIL Power Foam to soften the deposits before cleaning the screen with an abrasive pad. Reinstall the screen and test the trimmer.

Replace the screen altogether if it’s excessively plugged with carbon.

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) Switch To A Better Two-Stroke Oil

Low-quality oil that leads to heavy carbon is often to blame for most of the problems on this list.

Using a good two-stroke oil that burns cleanly and helps prevent carbon deposits is one of the easiest maintenance practices you can perform to ensure your gas trimmer starts easily, runs well and last for years.

AMSOIL SABER Professional Synthetic 2-Stroke Oil withstands high heat to fight carbon in gas string trimmers and other two-stroke equipment. It’s tested and proven at any mix ratio up to 100:1, offering the convenience of one mix ratio for all your equipment. Plus, it’s formulated with gasoline stabilizer to help keep fuel fresh during short-term storage.

The images here show AMSOIL SABER Professional’s superior cleanliness properties. It’s just one reason professional landscapers, like Duluth Lawn Care, only trust AMSOIL products.

Follow the gas trimmer troubleshooting guidelines on this list to get your string trimmer back up and running…and to give your shoulder a break.

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AMSOIL Technical Writer and avid avid DIYer with 12 years in the synthetic lubricants industry, who enjoys making technical topics in the automotive, powersports and industrial markets easy to understand.

STIHL FS 91 R Weed Eater Review

While the work doesn’t really slow down at this time of the year in Florida, those of us who review tools for a living had a great opportunity to put 10 of the OPE industry’s best string trimmer models up against each other. We wanted to see which brand offered the best performance, ergonomics, features, runtimes, and value. If that sounds like something you’d be interested in, you can read about the testing procedures and results here.

One of the things we like to do after a shootout is to go back to each individual model we tested and take a closer look at it. Today, we’re taking another gander at our 4th-place finisher, the STIHL FS 91 R String Trimmer. In typical STIHL fashion, this trimmer offers a lot of Pro-level performance and runtime. So, where’s the proverbial chink in the armor here? Let’s take a look at the many things that this piece of equipment does right, and the few things that could use some refinement.


To say that the ergonomics of the STIHL FS 91 R could use a lot of work would be overstating the case by a large margin. The truth is that we’ve got a bit of a mixed bag here. On the one hand, STIHL sinks a lot of effort into designing tools that lawn care Pros keep coming back to, and with good reason. The STIHL lineup has a reputation for comfortability, among other things. And this trimmer feels comfortable enough: it has the rubberized handles, an easy-to-access throttle, and an appropriately long shaft that won’t force too many people to lurch around the yard.

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On the other hand, STIHL makes some tradeoffs with this model. It has the largest tank of any of the models we tested at 24 oz, but that kind of fuel capacity adds a lot of weight with a full tank of gas. While the trimmer weighs in at 12.1 lbs on an empty tank, practically speaking, you’ll start your day lugging around almost 14 lbs.

Noise is another complaint we had with this model, and it comes as something of a surprise, as STIHL generally handles sound output very well. The FS 91 R measured 106 dB(A) at the ear and 80 dB(A) at 50 ft. Far and away the loudest model we tested, this string trimmer would benefit from a better muffler. Of course, many pros will throw on ear protection, so this complaint could be a minor one depending on who uses it.

Fuel Efficiency

Referring back to that tradeoff, here’s the good news: 24 oz. of fuel should last you a while. Making fewer trips back to the truck to refuel will ultimately save you time, and consequently, money. And, with a full tank, the STIHL FS 91 R will last for just over an hour before you’ll need to stop to refuel.

Granted, the FS 91 R was not the most fuel-efficient model we tested. An initial efficiency test, whereby we ran 8 oz of fuel through each model, proved that most of our string trimmers could run for longer on the same amount of fuel. The STIHL FS 91 R finished in 7th place here. So, for whatever it’s worth, you’ll have to stop less often for refueling, but you’re not getting the best fuel economy with this model.

It also bears mentioning that, while the STIHL FS 91 R does feature a 4-stroke engine, it still requires premix fuel. We wish this wasn’t the case, but such is life sometimes. Anyway, for our shootout, we used TruFuel 50:1 Premix fuel to keep things running optimally.

Feature Set

Now that we’ve got all the major criticisms out of the way, let’s look at where the STIHL FS 91 R really excels. STIHL has jam-packed this string trimmer full of features. In fact, it finished in joint 1st-place in this department. Some of the more notable features include a serviceable head, a one-piece straight shaft, a toolless filter change system, and a simplified 3-stage startup procedure.

STIHL FS 91 R String Trimmer Features

  • Reduced-emission engine technology
  • Built in the U.S.A.
  • Shoulder strap/carrying system
  • Adjustable front handle
  • Fully-lined drive shaft for less vibration
  • High-tech polymer housing
  • Protected choke knob
  • Start/run/stop controls on handle
  • Throttle trigger interlock
  • Air filter cover
  • Protected control cables
  • Heavy-duty steel-on-steel clutch
  • Protected spark arrestor muffler


Performance is where the proverbial rubber meets the road. A feature-rich string trimmer that struggles with actual trimming doesn’t do anyone a whole lot of good. But, we fitted all of our string trimmers with 0.095 Echo Black Diamond Trimmer Line, and then went to town in some of the thickest grass that Florida has to offer: overgrown Bahia.

The STIHL FS 91 R mowed through a 5′ x 5′ square of this tough grass in 13.5 seconds. For comparison, the fastest model we tested accomplished the same feat in just under 11 seconds.

We also ran these trimmers through a thinner, more typical patch of grass. Here, the FS 91 R finished in just under 9 seconds. Our fastest high torque model handled this same task in just under 8 seconds.

The differences between most of the models we tested were negligible. That said, in overall performance, the STIHL FS 91 R finished 5th.


As far as value goes, STIHL does some pretty commendable work with this model. Not only does this model offer some of the best features and runtimes we tested, but it also comes with a 2-year limited warranty. At 299, the STIHL FS 91 R retails for less than many of the models we tested. We’re not necessarily used to this with the STIHL brand, as they usually fall more on the expensive end of the pricing spectrum. But, in this case, only the Ryobi and Troy-Bilt models cost less. Ultimately, between its features, performance, warranty, and price point, we feel that the STIHL FS 91 R presents a really solid value.

The Bottom Line

We’re used to STIHL making well-engineered and comfortable lawn tools that a lot of Pros swear by. The FS 91 R will likely fit into that same mold for a lot of folks, but we feel like STIHL threw us a curveball or two with this model. For one thing, it’s a bit heavier and louder than we expected from this brand. And, while the runtime on the FS 91 R is excellent, its actual fuel efficiency is a little bit lackluster.

weed, eater, start, this, stihl, fuel

However, we’re also not necessarily used to seeing STIHL products priced lower than too many of their competitors. It performs as well as we’ve come to expect from the brand, and because of its large tank, the FS 91 R runs for longer than any of its competitors without needing to refuel. Plus, it includes some excellent features, like a serviceable head and an auto-choke.

Ultimately, the STIHL FS 91 R finished our shootout in an overall 4th place, and only by the slimmest of margins. For those guys already working with STIHL products, rest assured that this won’t be the string trimmer that disappoints you. And, for those folks who are looking to work with the brand, the STIHL FS 91 R should show why so many pros stick with STIHL products.

String Trimmer Gas To Oil Ratio By Brand

Finding the correct gas to oil ratio for a string trimmer is not as easy as it should be. Although the industry is moving slowly toward a consistent 50:1 ratio recommendation, there are still variations between manufacturers.

Recommended gas to oil ratios for the most common string trimmer brands:

BrandGas To Oil Ratio
STIHL 50:1
Echo 50:1
Husqvarna 50:1
McCulloch 40:1
Craftsman 40:1
Ryobi 50:1

It’s important to note that some brands have models that use different ratios than those stated above. It gets ridiculous with some brands that have two or three different recommended ratios depending on the model of your equipment.

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As such, it’s always best practice to check the owner’s manual but these represent the bulk of each brand’s offerings.

STIHL doesn’t mix words when it comes to mixing gas and oil. They maintain a consistent 50:1 ratio with all of their 2-stroke equipment (source). I appreciate this level of consistency.

Note that older owner manuals for Echo products recommended strange, sometimes impossible gas to oil ratios such as 32:1. But in 2019 Echo revised their recommendations to allow for a 50:1 ratio for all Echo two-stroke engines using their branded two-cycle oils (source – see page 23).

Husqvarna’s older manuals used to call for a 40:1 ratio. I contacted Husqvarna for clarification and they verified that newer models all use 50:1 but some older models do in fact require 40:1. I did not receive any specific guidance on the year that this change occurred so I recommend checking your owner’s manual if you have an older model string trimmer just to be sure.

McCulloch equipment uses 40:1 per the Husqvarna website (source).

Virtually every model of Craftsman string trimmer that I looked at used 40:1 gas to oil ratios, though I was unable to find a definitive source that could confirm that this is true of all of their equipment.

Ryobi, though best known these days for their battery-powered tools, does still maintain a few gas model trimmers. They currently recommend a 50:1 ratio for their 2-stroke trimmer engines (source).

Does Gas To Oil Ratio Really Matter?

Two-stroke engines are designed to run on specific gas to oil ratios. Straying from the manufacturer’s recommendations can result in various engine problems and potential damage.

Examples of problems that can be encountered from inaccurate gas to oil ratios include:

But these are just symptoms. The real issue is the potential damage that can be caused to the engine itself.

A two-stroke engine requires lubrication directly in the fuel since it does not have a separate oil compartment to draw lubrication from. Using an inaccurate gas to oil ratio means you are either not providing enough lubrication, or you are giving it too much.

Either way, the motor will not run as efficiently as it is designed to if you alter fuel ratio from the manufacturer recommendations.

Can I Use 4-Stroke Oil Instead Of 2-Stroke?

You should never use 4-stroke oil in a 2-stroke engine. 4-stroke oil will gum up over time when mixed directly with the gas, causing longterm motor issues and potential damage.

Some people have claimed success with using 4-stroke oil but short-term testing does not allow for adequate evaluation of longterm impact. There is absolutely no reason that you should place your 2-stroke equipment at risk with this unproven theory.

Should I Buy Premixed Fuel Or Just Mix The Gas And Oil Myself?

For ease of use, premium ingredients, and precise gas to oil ratios, commercially-prepared premixed fuel is a better solution than mixing the gas and oil yourself.

I spent years mixing my own fuel, priding myself on money saved in the short term. In the end, I was being penny-wise but pound-foolish. The gas I was using was from the pump at a local station. Pump fuel contains ethanol which can damage small engines.

In my case, it caused the inner lining of the fuel lines to dissolve resulting in a lot of work rebuilding the carburetor and trying to get the darn thing to run again.

I made the transition to premixed fuel and have never looked back. There are several quality products available on the market and I’ve already done a lot of the research by comparing the best premixed fuels so have a look at that before buying.

And yes, premixed fuel does cost more when you look strictly at price but you have to factor in that you are getting ethanol-free gas mixed precisely with 2-stroke oil and extended shelf life that far exceeds a homemade mix.

Where Can I Find My String Trimmer Manual Online?

The first place to look for a string trimmer manual online is the manufacturer’s website. Companies generally provide a directory of owner manuals in PDF format available for download.

Here are the direct links to owner manual directories for several leading string trimmer manufacturers:

ManufacturerOwner Manual Directory
STIHL (Link)
Echo (Link)
Husqvarna (Link)
McCulluch (Link)
Craftsman (Link)
Ryobi (Link)

If your brand is not listed, try a simple Google search. Any reputable manufacturer will have an online directory that is usually searchable. You can search for your specific string trimmer model and view or download the owner’s manual.

This manual will provide a definitive answer on the correct gas to oil ratio for your specific string trimmer model.


String trimmer gas to oil ratios vary by manufacturer but most are moving toward a 50:1 ratio. It is likely that in the years ahead this will become the defacto gas to oil ratio for 2-stroke yard tools.

In the mean time, it’s important to make sure that you are abiding by the recommended mix ratio specific to your brand and model. This will ensure that your string trimmer runs as efficiently as possible and receives the proper amount of lubrication, preventing damage and poor performance.

Paul has a two-acre yard on red clay soil in Southeast Texas. He knows exactly what the challenges are to nurturing a thriving yard in difficult soil.Paul takes a practical approach to yard improvement and enjoys putting best practices and “golden rules of lawn care” to the test.

What is 50:1 ratio Oil on Weed Trimmer

I have 2 cycle (?) STIHL FS 110R, and it says to give 50:1 ration oil and gasoline. What is this. I mean how do I mix it up? I used to just get an one bottle of tiny oil into 1 gallon of gasoline tank and mix with a gasoline shake it and good to go. now what do I have to do differently? ( I also have STIHL FS85R, and Craftsman chain saw) that I use and use the same mix to operate them. STIHL 110R is new stock I just got from some one and now readilng the manual on it and found this 50:1 ration thing.

Oh, also is it true you can get WARNING yellow operation sticker that goes to the shaft on my STIHL weed trimmer fre at any STIHL dealer??


Look at the small bottles in the picture. That’s two stroke oil. They have different sizes for different gas cans. If you use a 2.5 gallon can you use a smaller bottle than a 5 gallon gas can. Your STIHL dealer will sell you the right size bottle for the gas can you use.

Once upon a time, there was a lonely oxygen atom. The oxygen atom met two little hydrogen atoms, and suggested they try a threesome. They did, and everybody got wet. The end.


50:1 means for every 50 ounces of gasoline you need to mix in 1 ounce of oil. If you are using a 1 gallon gas can you would take 128 ounces (1 gal) divided by 50 = 2.56.

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I have 2 cycle (?) STIHL FS 110R, and it says to give 50:1 ration oil and gasoline. What is this. I mean how do I mix it up? I used to just get an one bottle of tiny oil into 1 gallon of gasoline tank and mix with a gasoline shake it and good to go. now what do I have to do differently? ( I also have STIHL FS85R, and Craftsman chain saw) that I use and use the same mix to operate them. STIHL 110R is new stock I just got from some one and now readilng the manual on it and found this 50:1 ration thing.

Oh, also is it true you can get WARNING yellow operation sticker that goes to the shaft on my STIHL weed trimmer fre at any STIHL dealer??

Read the oil bottle you use to fix for your other stuffs, it should say 2.6 oz. You mix 2.6 oz to one gallon, you get 50:1. This will be the same amount of oil for you old stuff and the new 4Mix trimmer. The thing more important is you should use STIHL Ultra ( white bottle) or Amsoil for the 4Mix. Synthetic is better for your older stuff anyway. Just mix 2.6 oz bottle of STIHL Ultra to one gallon of 89 octane or higher gas, Shake it good and you are good to go.

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