How to Start a Stihl Fs45 Weed Eater? Quick Start Tips

Introduction

The Stihl FS45 is one of the most popular lightweight string trimmers on the market, favored by homeowners and landscaping professionals alike for its balance of power, durability, and ease of use. If you’ve recently purchased this model or pulled it out of storage after a long winter, you may be wondering How to Start a Stihl Fs45 Weed Eater? especially if you’re new to operating 2-stroke gas-powered outdoor equipment. Unlike electric trimmers that start with the flip of a switch, the FS45 requires a few specific steps to get the engine running smoothly, and small mistakes like using the wrong fuel mix or skipping safety checks can lead to frustrating starting issues. This comprehensive guide walks you through every step of starting your FS45, from pre-start safety checks to troubleshooting common problems, so you can get back to trimming weeds and edging lawns in no time. We’ll cover both gas-powered and electric FS45 variants, share maintenance tips to reduce future starting headaches, and break down common mistakes to avoid so you can keep your trimmer running reliably for years.

Understanding Your Stihl FS45 Weed Eater Core Components

Before you attempt to start your FS45, it’s helpful to familiarize yourself with the key parts of the trimmer that play a role in the starting process. Knowing what each component does will make the step-by-step instructions easier to follow, and help you troubleshoot issues if the engine fails to start. The core components you’ll interact with when starting the FS45 include:

• Fuel tank: Located on the side of the trimmer, this holds the 2-stroke fuel mix for gas-powered models. Most FS45 units have a 0.8-liter (27 fl oz) tank capacity, enough for 30-45 minutes of runtime depending on throttle use.
• Primer bulb: A small rubber bulb next to the fuel tank that you press to pump fuel from the tank into the carburetor before starting. This helps get fuel to the engine quickly, especially when the trimmer has sat unused for a while.
• Choke lever: A small switch on the side of the carburetor that controls the air-to-fuel ratio. For cold starts, the choke is closed to enrich the fuel mixture, making it easier to start the engine. For warm starts, the choke is left open to avoid over-fueling.
• Stop switch: A toggle switch on the handle that controls power to the engine. It is usually marked with a checkmark (on) and an X (off). If the switch is off, the engine will not start regardless of other settings.
• Throttle trigger: The trigger on the handle that controls the engine speed. Most FS45 models have a lock-off button next to the trigger that you must press before squeezing the throttle to start the engine, as a safety feature to prevent accidental activation.
• Starter cord: The pull cord on the top of the engine that you yank to start the combustion process. The cord is attached to a recoil spring that spins the engine’s crankshaft when pulled correctly.
• Air filter: A small foam or paper filter that prevents dirt and debris from entering the engine. A clogged air filter can restrict airflow and make starting difficult, so it’s important to check it before each use.

For electric FS45 variants, you’ll also interact with a power cord or battery port, and the starting process is far simpler, as we’ll cover later in this guide.

Pre-Start Safety Protocols You Should Never Skip

Before you even touch the starter cord, taking a few minutes to complete pre-start safety checks will protect you from injury and prevent damage to your trimmer. These steps are non-negotiable, especially for gas-powered models that have hot engine components and flammable fuel:

1. Wear appropriate personal protective equipment (PPE): Always wear safety glasses or goggles to protect your eyes from flying debris, hearing protection to guard against engine noise, sturdy work boots with steel toes to protect your feet from the cutting line, long pants to prevent cuts from the trimmer head, and work gloves to protect your hands from vibration and sharp edges.
2. Inspect the work area: Clear the area you’ll be trimming of rocks, sticks, toys, and other debris that could be thrown by the cutting line. Make sure there are no bystanders or pets within 50 feet of the trimmer before starting.
3. Check the cutting head: Make sure the cutting line is properly installed and not tangled, and that the trimmer head guard is securely attached. A loose or damaged cutting head can cause serious injury if it detaches while the engine is running.
4. Check for fuel leaks (gas models only): Inspect the fuel tank, fuel lines, and carburetor for any signs of cracks, leaks, or fuel spills. If you smell gas or see visible leaks, do not attempt to start the trimmer until the issue is fixed, as fuel leaks pose a fire hazard.
5. Verify the trimmer is on stable ground: Place the FS45 on flat, level ground with the cutting head pointing away from your body and any obstacles. Never start the trimmer while holding it, as it can kick back when the engine fires.

Skipping these safety steps can lead to preventable injuries, so make them a habit every time you use your FS45.

Fuel Mixing and Preparation for Gas-Powered FS45 Units

One of the most common reasons users struggle with starting their Stihl FS45 is using the wrong fuel or an improper fuel mix. The FS45 is equipped with a 2-stroke engine, which requires a mixture of unleaded gasoline and 2-stroke engine oil to run. Unlike 4-stroke engines that have a separate oil reservoir, 2-stroke engines rely on the fuel mix to lubricate internal components, so using the wrong ratio or type of fuel can cause starting issues, poor performance, and even permanent engine damage.

2-Stroke Fuel Mix Ratios for the FS45

Stihl recommends a 50:1 fuel-to-oil ratio for all FS45 models, meaning you need 50 parts gasoline to 1 part 2-stroke oil. This ratio provides the right balance of lubrication and combustion for the engine. To mix fuel correctly:

• Use only fresh, unleaded gasoline with an octane rating of 89 or higher. Ethanol-free gas is highly recommended, as ethanol can break down over time and cause clogs in the carburetor. If you must use ethanol-containing gas, choose a blend with no more than 10% ethanol (E10), and avoid any gas with higher ethanol content.
• Use only Stihl-approved 2-stroke engine oil, or a high-quality TC-certified 2-stroke oil designed for air-cooled engines. Never use automotive motor oil, as it is not formulated for 2-stroke engines and will cause carbon buildup and starting issues.
• For a 50:1 mix, add 2.6 fluid ounces (77 ml) of 2-stroke oil to 1 gallon (3.8 liters) of gasoline. If you’re mixing a smaller batch, add 0.8 fluid ounces (24 ml) of oil to 1 quart (0.95 liters) of gas. Shake the mixture thoroughly in an approved fuel container to ensure the oil is fully blended with the gas before pouring it into the FS45’s fuel tank.

Never mix more fuel than you plan to use in 30 days, as 2-stroke fuel mix can degrade over time, especially if it contains ethanol. Old fuel is one of the top causes of starting problems for Stihl trimmers, so it’s worth mixing small batches regularly rather than storing large quantities of pre-mixed fuel.

Step-by-Step Guide: How to Start a Cold Stihl FS45 Weed Eater

A “cold start” refers to starting the FS45 after it has sat unused for 4 or more hours, or when the engine is at room temperature. Cold starts require a richer fuel mixture, so you’ll need to use the choke and press the primer bulb more times than you would for a warm start. Follow these steps exactly to start a cold FS45:

1. Position the trimmer correctly: Place the FS45 on flat, level ground with the cutting head pointing away from your body and any obstacles. Make sure the throttle lock-out button is not engaged, and the stop switch is in the “off” position to start.
2. Turn on the stop switch: Flip the stop switch on the handle to the “on” position, marked with a checkmark or the number 1. If the switch is off, the engine will not receive power and will not start.
3. Prime the carburetor: Press the primer bulb slowly 6 to 8 times. You should see fuel begin to fill the bulb after 3-4 presses. Do not press the bulb more than 8 times, as over-priming will flood the engine with fuel and make starting impossible until the excess fuel is cleared. The primer bulb pumps fuel from the tank into the carburetor, giving the engine the fuel it needs to ignite when you pull the starter cord.
4. Set the choke to the cold start position: Move the choke lever to the fully closed position, usually marked with a snowflake icon or the letter “C”. This closes off the air intake to the carburetor, creating a richer fuel-to-air mixture that is easier to ignite when the engine is cold.
5. Pull the starter cord: Grip the starter handle firmly and pull the cord slowly until you feel resistance, which indicates the piston is at the top of its compression stroke. Then pull the cord firmly and quickly in one smooth motion. Do not yank the cord violently, as this can damage the recoil spring. Pull the cord 3 to 4 times, waiting 2 seconds between pulls to let the engine turn over.
6. Adjust the choke if needed: If the engine sputters or makes a “chugging” sound but does not start after 4 pulls, open the choke lever halfway to reduce the fuel mixture slightly. Pull the starter cord 2 to 3 more times. The engine should start within a few pulls once the mixture is adjusted correctly.
7. Warm up the engine: Once the engine starts, let it idle for 30 seconds with the choke still partially closed. This allows the engine to reach optimal operating temperature before you put it under load.
8. Disengage the choke: Gradually open the choke lever fully to the “run” position, marked with a sun icon or the letter “R”. If you open the choke too quickly, the engine may die from a lack of fuel. Once the choke is fully open, squeeze the throttle trigger once to raise the idle speed, and your FS45 is ready to use.

If the engine does not start after 8 total pulls, stop and troubleshoot common issues like old fuel, a clogged air filter, or a flooded engine, which we cover later in this guide.

Step-by-Step Guide: Starting a Warm Stihl FS45 Weed Eater

A warm start is used when you’ve recently turned off the FS45 (within the last 4 hours) and the engine is still warm from use. Warm starts require less fuel enrichment, so you won’t need to use the choke or press the primer bulb as many times. Follow these steps for a quick warm start:

1. Position the trimmer on stable ground: Just like with a cold start, place the FS45 on flat ground with the cutting head away from you and any obstacles.
2. Turn on the stop switch: Flip the stop switch to the “on” position as you would for a cold start.
3. Prime the carburetor lightly: Press the primer bulb 2 to 3 times only. Over-priming a warm engine will flood it, making starting difficult.
4. Leave the choke fully open: Ensure the choke lever is in the fully open “run” position. Using the choke on a warm engine will add too much fuel to the mixture, causing the engine to sputter and die.
5. Pull the starter cord: Pull the starter cord firmly 1 to 2 times. The warm engine should start almost immediately. If it does not start after 2 pulls, check for issues like a clogged air filter or old fuel.
6. Check idle speed: Once the engine starts, let it idle for 10 seconds before squeezing the throttle to engage the cutting head. The FS45’s idle speed should be steady without stalling; if it stalls, adjust the idle screw slightly clockwise to raise the idle speed.

Warm starts are much faster than cold starts, and following these steps will ensure you don’t flood the engine or cause unnecessary wear on the starter system.

Starting Electric or Corded Stihl FS45 Variants

While the standard Stihl FS45 is a gas-powered 2-stroke trimmer, Stihl also offers corded electric and battery-powered variants of the FS45 line that have a far simpler starting process, with no fuel mixing or priming required. If you own one of these electric models, follow these steps to start it:

1. For corded models: Plug the trimmer’s power cord into a grounded 120V outdoor outlet. Inspect the cord for any cuts, fraying, or damage before plugging it in; never use a damaged cord, as it poses an electric shock hazard. Make sure the cord is not tangled around the cutting head or your feet.
2. For battery-powered models: Insert the fully charged Stihl battery into the battery port on the trimmer handle until you hear a click. Make sure the battery is fully charged, as a low battery may not provide enough power to start the trimmer under load.
3. Turn on the stop switch: Flip the stop switch on the handle to the “on” position.
4. Disengage the throttle lock-off: Press the throttle lock-off button next to the trigger, then squeeze the throttle trigger firmly. The motor will start immediately, with no pulling or priming required.
5. Adjust speed as needed: Use the variable speed dial on the handle (if equipped) to set the cutting speed to match the thickness of the weeds you’re trimming.

Electric FS45 models are ideal for small to medium yards where a power outlet is easily accessible, and they require far less maintenance than gas-powered variants. However, they have less power than gas models, so they may struggle with thick, woody weeds or tall grass.

Troubleshooting Common Starting Issues

Even if you follow all the starting steps correctly, you may occasionally run into issues that prevent your FS45 from starting. Most starting problems are easy to fix with basic troubleshooting. Below are the most common issues and their solutions:

Dealing with Flooded FS45 Engines

A flooded engine is the most common starting issue for gas-powered FS45 models, and it occurs when too much fuel enters the carburetor, usually from over-priming the bulb or leaving the choke on for too long during a start attempt. Signs of a flooded engine include a strong smell of gas coming from the trimmer, the starter cord pulling very easily (no resistance), and the engine sputtering but not starting. To fix a flooded engine:

1. Turn the stop switch to the “off” position.
2. Move the choke lever to the fully open position.
3. Hold the throttle trigger fully open while pulling the starter cord 5 to 6 times. This clears excess fuel from the carburetor and engine cylinder.
4. Wait 5 minutes for the excess fuel to evaporate, then follow the warm start steps to restart the trimmer.

If the engine is still flooded after this process, you may need to remove the spark plug and dry it with a clean cloth before reinstalling it.

Other common starting issues include:

• No spark from the spark plug: Remove the spark plug and check for a spark by holding the metal part of the plug against the engine block while pulling the starter cord. If there is no spark, the spark plug may be fouled or worn out, and needs to be replaced. The recommended spark plug for the FS45 is the NGK BPMR7A, with a gap of 0.02 inches (0.5 mm).
• Clogged air filter: A dirty air filter will restrict airflow to the engine, making it impossible to start. Remove the air filter cover and clean the foam filter with warm soapy water, or replace it if it is torn or heavily soiled.
• Old or bad fuel: If your fuel mix is older than 30 days, or contains high levels of ethanol, drain the tank and mix fresh fuel before attempting to start again. Old fuel can varnish the carburetor jets and prevent fuel from reaching the engine.
• Clogged fuel line: If fuel is not flowing from the tank to the carburetor, the fuel line may be clogged with debris or varnish from old fuel. Disconnect the fuel line and clear any clogs, or replace the line if it is cracked or damaged.

Maintenance Tips to Keep Your FS45 Starting Easily

Regular maintenance is the best way to avoid starting issues and extend the life of your Stihl FS45. Follow these simple maintenance tasks after every 5 hours of use, or at least once per month if you use the trimmer infrequently:

• Clean the air filter: Remove the air filter cover and wash the foam filter with warm, soapy water. Squeeze out excess water and let it dry completely before reinstalling. If the filter is torn or heavily soiled, replace it with a new Stihl air filter. A clean air filter ensures proper airflow to the engine, making starting easier and improving performance.
• Clean the spark arrestor: The spark arrestor is a small metal screen in the muffler that prevents sparks from exiting the exhaust. Over time, it can become clogged with carbon buildup, which restricts exhaust flow and makes starting difficult. Remove the spark arrestor and clean it with a wire brush, or replace it if it is damaged.
• Lubricate the gear head: Apply a small amount of Stihl gear lubricant to the grease fittings on the gear head after every 2-3 hours of use. This reduces wear on the cutting head components and prevents the engine from working harder than necessary.
• Clean the fuel system: If you store the FS45 for more than 30 days, drain the fuel tank and run the engine until it stalls to clear all fuel from the carburetor. This prevents old fuel from varnishing the carburetor jets and causing starting issues when you use the trimmer again.
• Sharpen or replace the cutting line: A dull or damaged cutting line will make the engine work harder, putting extra strain on the starter system. Replace the line regularly to keep the engine running smoothly and reduce starting effort.

Best Practices for Long-Term FS45 Performance

In addition to regular maintenance, following these best practices will keep your FS45 starting easily for years:

• Always use fresh, high-quality fuel mix, and never store the trimmer with fuel in the tank for more than 30 days. If you need to store it long-term, drain the tank completely.
• Store the FS45 in a cool, dry place out of direct sunlight. Extreme temperatures can degrade fuel and damage rubber components like the primer bulb and fuel lines.
• Check the spark plug gap every 6 months, and replace the spark plug annually to ensure a strong spark for starting.
• Never pull the starter cord to its full extension, as this can break the recoil spring. Pull the cord only until you feel resistance, then pull firmly in one smooth motion.
• If you notice the starter cord is frayed or the recoil spring is not working properly, replace the starter assembly immediately to avoid a complete starter failure that will leave you unable to start the trimmer.

Common Mistakes to Avoid When Starting Your FS45

Many starting issues are caused by simple, avoidable mistakes. Avoid these common errors to keep your FS45 running reliably:

• Using the wrong fuel mix: Never use pure gasoline, diesel, or car motor oil in your FS45. Pure gas will seize the engine due to lack of lubrication, and car oil is not formulated for 2-stroke engines and will cause carbon buildup. Always use a 50:1 mix of unleaded gas and TC-certified 2-stroke oil.
• Over-priming the carburetor: Pressing the primer bulb more than 8 times for a cold start, or more than 3 times for a warm start, will flood the engine with fuel. If you flood the engine, follow the steps in the flooded engine troubleshooting section to clear the excess fuel before trying to start again.
• Leaving the choke on for warm starts: Using the choke when the engine is already warm will add too much fuel to the mixture, causing the engine to sputter and die. Always leave the choke fully open for warm starts.
• Using old or ethanol-heavy fuel: Fuel that is older than 30 days, or contains more than 10% ethanol, will degrade and clog the carburetor. Always mix fresh fuel, and use ethanol-free gas if possible for best performance.
• Skipping safety checks: Failing to inspect the cutting head, work area, or fuel lines before starting can lead to injury or trimmer damage. Always complete the pre-start safety checks outlined earlier in this guide.

Frequently Asked Questions

Why won’t my Stihl FS45 start after sitting all winter?

If your FS45 has sat unused all winter, the most common causes of starting failure are old fuel left in the tank, a clogged carburetor from varnished fuel, or a dead spark plug. First, drain all old fuel from the tank and mix fresh 50:1 fuel. Clean the air filter and spark plug, then try starting the trimmer using the cold start steps. If it still won’t start, the carburetor may need to be cleaned or rebuilt to remove varnish buildup from old fuel.

Can I use regular unleaded gas in my FS45?

Yes, you can use regular unleaded gasoline with an octane rating of 89 or higher in your FS45, as long as it is mixed with the correct amount of 2-stroke oil at a 50:1 ratio. Ethanol-free gas is strongly recommended, as ethanol can break down over time and cause clogs in the carburetor. If you use gas with ethanol, choose a blend with no more than 10% ethanol (E10), and avoid any gas with higher ethanol content, as it can damage the engine’s internal components.

How do I know if my FS45 is flooded?

Signs of a flooded FS45 engine include a strong smell of raw gasoline coming from the muffler or fuel tank, the starter cord pulling very easily with no resistance (indicating no compression from excess fuel), and the engine sputtering or making a “chugging” sound when you pull the starter cord but not starting. If you notice these signs, follow the flooded engine troubleshooting steps: turn the choke off, hold the throttle open while pulling the starter cord 5-6 times to clear excess fuel, wait 5 minutes, then try starting again using the warm start procedure.

Do I need to run the FS45 out of fuel before storing it?

Yes, if you plan to store your FS45 for more than 30 days, you should drain all fuel from the tank and run the engine until it stalls to clear fuel from the carburetor. This prevents old fuel from varnishing the carburetor jets and causing starting issues when you use the trimmer again. If you only plan to store it for 2-3 weeks, you can leave fuel in the tank, but make sure it is fresh, ethanol-free fuel mixed at the correct 50:1 ratio.

How often should I replace the spark plug on my FS45?

Stihl recommends replacing the spark plug on your FS45 annually, or after every 100 hours of use, whichever comes first. You should also check the spark plug gap every 6 months to ensure it is set to the correct 0.02-inch (0.5 mm) specification. A worn or fouled spark plug will produce a weak spark, making starting difficult and causing poor engine performance. Replacing the spark plug is a simple, inexpensive maintenance task that will keep your FS45 starting easily.

Conclusion

Starting a Stihl FS45 weed eater doesn’t have to be a frustrating experience, even if you’re new to gas-powered outdoor equipment. By following the step-by-step instructions outlined in this guide, completing pre-start safety checks, using the correct 50:1 fuel mix, and performing regular maintenance, you’ll be able to start your FS45 quickly and reliably every time. Whether you’re performing a cold start after winter storage, a warm start mid-job, or using an electric FS45 variant, the process is straightforward once you know the correct steps. Remember to avoid common mistakes like over-priming, using old fuel, and skipping safety checks, and address starting issues promptly by troubleshooting common problems like flooded engines or clogged air filters. With proper care and maintenance, your Stihl FS45 will provide years of reliable service for all your weed trimming and edging needs. If you’ve ever asked How to Start a Stihl Fs45 Weed Eater? this guide has all the information you need to get the job done safely and efficiently.