Stihl FS 90 Weed Eater (Starting Step-by-Step)
To start a STIHL FS 90, check the guard, cutting attachment, fuel, and controls first. For a cold start, prime the fuel system, set the choke to cold start, engage the starting-throttle setting, and pull the starter until the engine fires. Open the choke and pull again. A warm engine usually starts with the choke open.
Does your FS 90 refuse to start when the yard work cannot wait? A steady routine helps you avoid common starting errors and spot safety problems before the cutting head turns. The FS 90 is a 28.4 cm³ STIHL 4-MIX brushcutter. Its fuel must be gasoline mixed with two-cycle engine oil at 50:1, not straight gasoline.
Controls can differ by model year or market. Check the instructions supplied with your exact machine before starting, especially for the starting-throttle setting, fuel mix, and approved cutting attachments.
Equipment Setup and Safety Preparation
This preparation checks that the FS 90 is fueled, assembled, and safe to start. It matters because the correct starting steps cannot make an unsafe setup safe. Before pulling the starter, confirm the attachment and its guard are approved for your exact version, controls work, and people are clear of the work area.
Check the machine before starting
Wear eye and face protection, hearing protection, sturdy footwear, long trousers, and gloves. Keep people and animals at least 15 m (50 ft) away; allow more distance if the attachment or site could throw debris farther.
Set the machine on firm, level ground. Check that the cutting attachment is secure and undamaged, and that the correct deflector or guard is fitted. A damaged, loose, or incompatible guard can leave you exposed to high-speed debris. Stop here if anything looks wrong; do not run the machine until the approved parts are correctly installed.
Check that the throttle returns freely and the stop control works. Look for fuel leaks around the tank and fuel lines. Clear away loose objects, wire, stones, and other debris that the attachment could strike.
Confirm the fuel and controls
The FS 90 uses a 50:1 gasoline-to-two-cycle-oil mix, following the fuel and oil grades in the manual for your machine. Do not use straight gasoline or a mix you cannot verify; incorrect lubrication can cause severe engine damage. If you are unsure what is in the tank, do not start it.
The controls typically include an ignition or stop switch, throttle trigger and lockout, choke, and manual fuel-pump bulb. Their positions may vary. Find the labels and diagrams on your machine, then match them to its manual rather than guessing.
| Before starting | What to check | If it is not right |
|---|---|---|
| Fuel | Correct, verified 50:1 mix | Do not start until fuel is confirmed |
| Guard | Approved, secure, undamaged | Stop and fit the correct guard |
| Attachment | Approved for the FS 90 version; secure and sound | Do not use a loose or damaged part |
| Throttle and stop control | Throttle moves freely; stop control works | Have the control checked before use |
| Work area | At least 15 m (50 ft) from people and animals | Clear the area or increase the distance |
Next step: Start only after the machine, fuel, and work area pass these checks.
Starting and Field Landscaping Operation
Starting uses a different choke setting for a cold engine than for a warm one. In both cases, keep the cutting attachment clear of people, objects, and vegetation. Follow your machine’s manual for the exact control positions and starting-throttle setting, since details can vary across FS 90 versions.
Cold-start the FS 90 step by step
- Place the machine on firm ground. Keep the cutting attachment clear and make sure it cannot strike anything when the engine starts.
- Move the stop or ignition control to RUN.
- Press the manual fuel-pump bulb until fuel is visible, then press it several times as directed in your manual. Do not guess at a fixed number.
- Set the choke to the closed or cold-start position.
- Engage the starting-throttle setting as shown on the machine or in its manual. Hold the unit securely, with the attachment clear.
- Pull the starter handle until the engine fires. Use a smooth pull; do not extend the rope all the way or let the handle snap back.
- Move the choke to open or run, then pull again until the engine runs.
If it fires but stops, check that the choke is in the run position before trying again. Repeated pulls with the wrong choke setting may make starting harder. If the machine still will not start, stop and use the troubleshooting checks below rather than pulling indefinitely.
Warm-start the FS 90
For a warm engine, set the stop control to RUN and leave the choke open. Keep the attachment clear, then use the warm-start procedure in your manual. If it does not catch, do not immediately switch to a cold-start routine; first confirm the control positions and follow the manual’s instructions for a warm engine that will not start.
After starting, let the engine settle before moving into the work area. Keep both hands on the handles and use the harness supplied or specified for the machine. Keep the attachment below waist height and pointed away from people.
When the engine does not start
Use this order to check simple causes without adjusting the carburetor by guesswork.
| What you notice | Check first | Safe next step |
|---|---|---|
| No sign of firing on a cold start | RUN position, fuel bulb, choke, starting-throttle setting | Compare each position with the manual |
| Engine fires, then stops | Whether the choke was moved to open/run | Set it to run and follow the restart procedure |
| Fuel mix is uncertain | Fuel container and mixing record | Do not start; replace with verified, correct mix |
| Fuel smell or visible leak | Tank, cap, and nearby fuel lines | Stop; do not start until the leak is repaired |
| Attachment turns or seems unsafe at idle | Attachment, throttle, and controls | Stop the engine and arrange inspection |
The FS 90’s listed maximum engine speed is approximately 9,500 rpm and its idle speed approximately 2,800 rpm. Confirm specifications for your model and market in its manual. If the machine idles poorly or the attachment behaves unexpectedly, do not try to fix it by turning carburetor screws without the right service information.
Next step: If the correct start sequence fails, use the manual and check fuel and controls first; have persistent faults assessed by a qualified service provider.
Maintenance, Inspection, and Cutting-Attachment Care
Routine checks help you catch damage before it becomes a starting or safety problem. The right attachment and guard depend on the FS 90 version and the tool fitted. Use only combinations approved by STIHL for your machine, and keep the guard in place whenever the engine runs.
Match the attachment to its guard
A cutting attachment is the part that trims or cuts; the guard, also called a deflector, helps protect you from debris. Use the correct guard for the fitted head or blade, along with any other parts required by the manual. Do not assume that a guard approved for one attachment is suitable for another.
Before each use, check the attachment for cracks, bending, missing parts, or looseness. Check that the guard is secure and intact. If the guard is cracked, loose, or incompatible, stop and correct the setup before starting. Never operate without the required guard.
Inspect after use and before the next start
Stop the engine before checking, cleaning, changing, or clearing the attachment. Wait until it has stopped completely. If grass or string is wrapped around the attachment, do not pull it free while the engine is running.
After the machine cools, remove grass and debris, then look for fresh damage, loose parts, or fuel leaks. Follow the manual’s maintenance schedule for tasks such as air-filter and spark-plug service. Use the listed parts and procedures; a dirty filter or worn component may need attention, but symptoms alone do not prove which part has failed.
Next step: Record problems you notice, such as a leak or repeated hard starting, and address them before the next job.
Shutdown, Storage, and Seasonal Care
A safe shutdown prevents the cutting attachment from being set down while it is still moving. Seasonal care also starts with the fuel: the correct storage steps depend on your machine’s manual and the fuel you use. Do not rely on a general storage rule that conflicts with STIHL’s instructions for your model.
To stop the FS 90, release the throttle and let the engine idle briefly. Move the stop control to STOP. Wait until the cutting attachment has stopped fully before setting the machine down.
When the engine and attachment are cool, remove debris and inspect the guard and cutting parts again. Store the machine as directed in the manual, away from heat and ignition sources. Follow the manual’s fuel-storage instructions for the product and fuel in use. Before the next season, verify the fuel mix and inspect the attachment, guard, controls, and fuel system before starting.
Next step: Keep the manual with your seasonal equipment notes so the fuel and storage instructions are easy to check.
Field Scenarios and Quick Decisions
These scenarios show how to apply the starting and safety checks to common situations. They are practical examples, not reports about a specific FS 90 or repair. In each case, the manual for your exact model remains the final guide to control positions and approved parts.
- The engine is cold after winter storage: Verify the fuel rather than assuming it is usable, inspect for leaks and damage, then follow the cold-start sequence. If the fuel history is uncertain, do not run the machine until you have resolved it.
- The engine stopped during a short break: Treat it as a warm start if it is still warm. Use RUN with the choke open and the attachment clear. Follow the manual if it does not start.
- A new head or blade has been fitted: Do not start until you have confirmed it is approved for that FS 90 version and paired with its required guard. A change in attachment can change the guard required.
- The machine starts, but a guard is cracked: Stop the engine. Do not resume work until the correct, approved guard is fitted and secure.
Frequently Asked Questions
What fuel mix does a STIHL FS 90 use?
It uses gasoline mixed with two-cycle engine oil at 50:1. Follow the fuel and oil grades specified in your manual.
Can I start the FS 90 with straight gasoline?
No. Use the specified 50:1 mix. Straight gasoline can cause severe engine damage.
How do I cold-start an FS 90?
Set RUN, prime the bulb as directed, close the choke, engage the starting-throttle setting, and pull until it fires. Open the choke and pull again until it runs.
How do I warm-start an FS 90?
Set RUN, leave the choke open, keep the attachment clear, and use the warm-start procedure in your manual.
How many times should I press the fuel-pump bulb?
Follow the manual for your exact version. Press until fuel is visible, then press several times as directed; do not rely on an assumed count.
Why does it fire and then stop?
Check whether the choke was moved to open/run after the engine fired. If the issue continues, verify the fuel and control positions using the manual.
Can I use any trimmer head or blade?
No. Use only cutting attachments approved for your FS 90 version, with the guard specified for that attachment.
Can I run the FS 90 without its guard?
No. The required guard helps protect against thrown debris. Stop if it is damaged, loose, or incompatible.
How far should people stay away while I operate it?
Keep people and animals at least 15 m (50 ft) away. Increase the distance if the attachment or work conditions could throw debris farther.
What should I do if it still will not start?
Stop repeated attempts and check the manual, fuel, choke position, and controls. Do not adjust the carburetor by guesswork; seek qualified service for a persistent problem.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
