What Is The Stihl FS 120 Purge Bulb (Troubleshooting)
The STIHL FS 120 purge bulb is a small primer that moves fuel through the carburetor circuit and sends excess fuel back to the tank. If it will not fill, rebound, or move fuel, check the fuel, bulb, hoses, and tank pickup safely. Do not start the brushcutter if fuel is leaking.
A purge-bulb problem can look like a carburetor failure, but simple checks may reveal a loose hose, blocked fuel pickup, or damaged bulb. Finding the cause before repeated starting attempts helps protect both you and the engine. Think of the checks below as a low-cost first step before deciding whether the fuel system needs professional service.
What the FS 120 purge bulb does
The purge bulb, sometimes called a primer bulb, helps move fuel through the fuel system before starting. On the FS 120, it draws fuel through the pickup and carburetor circuit, then returns excess fuel to the tank. It is not a fuel-storage reservoir, and it does not replace the normal starting procedure.
Pressing and releasing the bulb moves fuel through connected passages. A serviceable bulb should generally spring back when released, and repeated presses should normally show fuel movement. The precise fill level and hose layout can vary by carburetor version, so do not assume the bulb must look completely full.
The FS 120 is a gasoline-powered brushcutter, not a chainsaw. Its displacement is 30.8 cm³. Chainsaw parts and instructions do not apply; use only a cutting attachment, guard, and harness approved for your specific FS 120 version.
FS 120 Purge-Bulb Setup and Safety Checks
Before inspecting the purge system, stop the engine and let it cool. Work outdoors, away from flames, cigarettes, sparks, or other ignition sources. Wear eye protection and fuel-resistant gloves. If you find wet fuel or smell a strong fuel odor, do not start the machine.
Begin with a visual check. Look for a split or cloudy bulb, cracked or disconnected hoses, loose connections, and signs of fuel around the tank or carburetor. Do not pull hard on small hose fittings; old fuel lines and plastic connections can be fragile.
Check that the tank contains fresh fuel mixed at 50:1 with quality air-cooled two-cycle engine oil that meets STIHL’s stated specification. Do not use straight gasoline. If you are unsure what is in the tank, do not guess or add more oil to compensate. Consult the applicable manual for your machine.
With the engine off, press and release the bulb several times while watching it and the visible hoses. Note whether the bulb rebounds and whether fuel moves. Do not force a stuck or brittle bulb.
| What you observe | Possible area to check | Safe next step |
|---|---|---|
| Bulb stays empty | Low fuel, blocked pickup, disconnected hose, or air leak | Check fuel level and visible hose connections |
| Bulb does not rebound | Damaged bulb or restricted line | Stop pressing; inspect for damage |
| Bulb fills but then collapses | Restricted fuel path or a line issue | Stop; avoid forcing it and seek service if the cause is not visible |
| Fuel leaks near bulb or hose | Split bulb, loose fitting, or damaged line | Do not start; repair the leak first |
| Fuel moves, but engine will not start | Starting procedure or another engine fault | Follow the manual’s cold- or warm-start steps |
These observations point to areas to investigate; they do not prove which part has failed. Hose routing and bulb appearance can differ by carburetor version. Use the correct FS 120 manual or have a qualified technician confirm the layout before disconnecting anything.
Troubleshooting a bulb that will not fill or move fuel
A bulb that stays empty does not automatically mean the bulb itself is faulty. Fuel may not be reaching it because of a low tank, a blocked pickup filter, a disconnected hose, or an air leak. Start with visible checks rather than opening the carburetor.
- Confirm the fuel level and condition. Use fresh, correctly mixed fuel. If the fuel is stale or the mixture is unknown, replace it according to the manual and local disposal rules.
- Inspect the tank pickup and lines. Look for a kink, crack, loose connection, or line that appears disconnected. Check the pickup area for an obvious obstruction, but do not tug on fragile parts.
- Watch the bulb while pressing it. A bulb that is split, stuck, or unable to rebound may need replacement. Do not keep pressing a damaged bulb.
- Stop if the cause is not visible. If the lines and bulb look sound but fuel still will not move, the problem may be inside the carburetor or in a less visible part of the fuel circuit. Have a qualified service technician test it.
Fuel leakage is a critical warning. A cracked bulb or loose hose can spill gasoline and may also let air enter the system. Air ingress can interfere with fuel delivery and may lead to lean running, which can damage the engine. Do not test for leaks with a flame or by running the machine. Contain any spill safely and repair the fault before starting.
Field Operation After Priming
Priming is only one part of starting an FS 120. Once the bulb and fuel system appear sound, refit any covers you removed and follow the starting steps in the manual for a cold or warm engine. Start outdoors in a clear area, with the machine positioned and controlled as the manual directs.
After starting, check that the engine idles steadily and responds normally to the throttle. The typical idle speed is about 2,800 rpm, but confirm the correct specification for your FS 120 version before making an adjustment. The cutting attachment should not move at idle.
Do not use the idle-speed screw to mask a fuel leak, a defective purge bulb, or a fuel-line fault. If the engine stalls, leaks fuel, or runs abnormally, stop it and investigate before using it again. Once checks are complete, stop the engine, let it cool, and clean up spilled fuel.
Fuel-System Maintenance and Carburetor Service
The carburetor meters fuel and air for the engine. Its internal passages and check valves help control fuel movement, but they are not usually visible during a basic external inspection. If the bulb and hoses look intact and the fuel is fresh, a fault inside the carburetor may require proper testing.
Avoid guessing at hose routing or removing carburetor parts without the correct manual and service skills. A misplaced hose can prevent fuel from moving as intended. If the bulb fills but the engine still will not start, that alone does not confirm a carburetor failure; ignition, air, and other fuel-system issues may also be involved.
I teach owners to separate what they can safely observe from what needs testing. For example, a visible split bulb is a clear reason not to start. A bulb that looks intact but will not move fuel is a symptom, not a diagnosis. In that situation, a qualified technician can check the pickup, lines, carburetor passages, and check valves without relying on guesswork.
Illustrative troubleshooting scenarios
These examples show how to use the checks above. They are practical scenarios, not reports of a particular FS 120 repair, and the correct hose layout can vary by version.
- The bulb remains empty, and a hose looks loose. Do not start the brushcutter. Confirm the routing using the applicable manual or have a technician inspect it. A loose connection can interrupt fuel movement or allow air into the system.
- The bulb rebounds, but the engine does not start. Rebound is useful information, but it does not prove the whole fuel system works. Confirm fresh, correctly mixed fuel and follow the manual’s starting steps. If the engine still will not start, stop repeated attempts and arrange a broader diagnosis.
- Fuel appears around the bulb after pressing. Treat this as a leak. Stop pressing, keep ignition sources away, and repair or replace the damaged part before starting.
The practical takeaway is to stop at the first sign of leakage or damaged parts. Do not keep trying to start the engine to see whether the problem clears.
Storage and Seasonal Fuel Care
Fuel left in a machine can become less suitable for use over time, depending on its age, storage conditions, and the fuel formulation. For storage steps and limits, follow the FS 120 manual and the fuel maker’s guidance rather than relying on a general time estimate.
Before seasonal storage, inspect the bulb and visible hoses for cracks or loose connections. Use the storage procedure specified in the manual. At the next use, check for leaks and confirm the fuel is fresh and correctly mixed before priming.
Conclusion
The FS 120 purge bulb moves fuel through the carburetor circuit and returns excess fuel to the tank. Check fuel, visible hoses, the pickup, and the bulb with the engine off. If fuel leaks, do not start the machine. If basic checks do not explain the problem, have the carburetor and fuel system tested rather than guessing at adjustments.
FAQs
What is the purge bulb on a STIHL FS 120?
It is a small primer that moves fuel through the carburetor circuit and returns excess fuel to the tank before starting.
Should the FS 120 purge bulb fill completely?
Not always. Bulb appearance can vary with the carburetor version and hose layout. Look for normal rebound and fuel movement rather than assuming a full-looking bulb is required.
Why does my FS 120 purge bulb stay empty?
Possible causes include low fuel, a blocked pickup, a disconnected or restricted hose, an air leak, or a fault in the carburetor circuit.
Why does the bulb not spring back?
It may be damaged or affected by a restricted line. Do not force it. Inspect for visible damage and seek service if the cause is unclear.
Can I start the FS 120 if fuel is leaking near the bulb?
No. Stop and repair the leak before starting. Fuel can ignite, and a leak may also let air into the fuel system.
Does the FS 120 use straight gasoline?
No. It uses unleaded gasoline mixed 50:1 with suitable air-cooled two-cycle engine oil, following STIHL’s stated specification.
Can I adjust idle speed to fix a purge-bulb problem?
No. Idle adjustment does not repair a leak, damaged bulb, blocked pickup, or faulty fuel line. Confirm the correct idle specification in the applicable manual.
When should a technician inspect the FS 120?
Ask a qualified technician if the bulb and visible lines look sound but fuel still will not move, or if the engine leaks, stalls, or runs abnormally.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
