Stihl FS 38 Won’t Prime (Primer Bulb Fixes)
If the FS 38 primer bulb will not fill, stay collapsed, or move fuel, check fresh fuel, the bulb, fuel lines, tank pickup filter, and tank vent in that order. Work with a cool engine and the switch at STOP. Replace only parts matched to your unit’s serial number and carburetor version; a bulb alone may not fix a blocked purge circuit.
What if the trimmer ran well at the end of last season, but now the bulb stays dry when you press it? That does not always mean the bulb itself has failed. A kinked line, blocked pickup filter, stale fuel, or fault inside the carburetor can stop the purge circuit from moving fuel.
I start with the simple checks before removing covers. This helps avoid replacing a sound bulb while missing a leak or restriction elsewhere. The FS 38 is a two-stroke line trimmer, not a chainsaw, so its parts and fuel instructions are specific to the trimmer. Use the manual and parts diagram for your unit’s serial number.
How the FS 38 primer works
The primer bulb is a hand-operated fuel pump. Pressing it moves fuel through the carburetor’s purge circuit and back toward the tank. The exact line routing and carburetor parts can vary by version, so use the diagram for your FS 38 rather than copying a route from another model.
Press the bulb at least five times when preparing to start, even if you can already see fuel in it. The bulb helps move fuel through the system; it does not confirm that the engine has spark, clean air, or a clear exhaust.
The FS 38 has a 27.2 cm³ two-stroke engine. Stihl specifies a 50:1 gasoline-to-Stihl two-cycle oil mix. Its fuel guidance calls for gasoline rated at least 89 AKI and no more than 10% ethanol. Check your own manual or label if its version differs. Do not use straight gasoline or guess the oil ratio; incorrect fuel can cause severe engine damage.
Diagnose the bulb before replacing parts
A quick, orderly inspection can separate a bulb fault from a fuel supply or carburetor problem. First check the fuel, then watch what the bulb does as you press it. A bulb that stays collapsed suggests a different fault from one that will not fill.
Work outdoors with the engine cool and the stop switch at STOP. Keep fuel away from flames, sparks, hot surfaces, and smoking materials. Wear eye and face protection, gloves, long trousers, sturdy footwear, and hearing protection when running the trimmer. Keep people and pets clear, following the manual’s distance guidance.
Check fuel and observe the bulb
Fresh, correctly mixed fuel and a sound tank cap are the first checks. Look for enough fuel in the tank, then press the bulb five times and note whether it fills, stays collapsed, or shows bubbles. Each result offers a clue, but none by itself proves which part has failed.
Use the table to guide the next inspection. Do not keep pulling the starter or run the trimmer if fuel is leaking or the bulb cannot prime.
| What you see | Possible causes to inspect | Next step |
|---|---|---|
| Bulb will not fill | Cracked bulb, loose or incorrectly routed line, air leak, blocked filter, or carburetor purge fault | Inspect the bulb and accessible lines; verify routing against the parts diagram |
| Bulb fills partly or shows bubbles | Loose connection, damaged line, low fuel, or restriction | Check line ends and tank pickup; add fresh mix if needed |
| Bulb stays collapsed | Blocked pickup or line, or restricted tank vent | Inspect for kinks and check the pickup; do not force the bulb |
| Bulb fills, but engine will not start | Another starting fault, or a carburetor issue | Follow the manual’s start procedure, then diagnose ignition, air, exhaust, and carburetion separately |
| Fuel appears outside the system | Split bulb, disconnected line, or failed connection | Stop; cool the engine and repair the leak before restarting |
If you smell fuel or see a wet spot, do not test for leaks with the engine running. A split bulb or disconnected line can let fuel reach a hot engine or exhaust, creating a fire risk.
Inspect the bulb, lines, filter, and vent
A fuel line carries fuel between the tank and carburetor. The pickup filter sits inside the tank and screens debris. A tank vent lets air replace fuel as it leaves the tank. A fault in any of these parts can interrupt priming, so look for damage and restrictions before blaming the carburetor.
Check accessible parts for cracks, hardening, swelling, kinks, loose ends, or wet fuel. If the bulb stays collapsed, inspect for a pinched or blocked line and a restricted pickup. If it will not fill, look closely for air leaks, disconnected ends, damaged lines, and incorrect routing.
Use the FS 38 parts diagram matched to the serial number and carburetor version to confirm the correct parts and line path. Do not assume that a universal bulb or another FS 38 version’s routing will fit. If the bulb is part of a carburetor assembly on your version, install the specified assembly rather than forcing a separate bulb into place.
Replace a damaged bulb or fuel line safely
Replacing a cracked bulb or damaged line can restore priming when the rest of the circuit is clear. The repair depends on your carburetor version and the correct parts. Take a photo before disconnecting any line, then follow the unit’s manual and parts diagram for removal and routing.
Before repair, let the engine cool completely, set the switch to STOP, and drain fuel into an approved container. Work outdoors and wipe up spills at once. Do not use a running engine to find a leak.
- Remove covers only as directed in the FS 38 manual.
- Photograph the existing line connections before removing anything.
- Check the parts diagram against the serial number and carburetor version.
- Replace the damaged bulb, line, or specified assembly. Seat each connection fully and route lines exactly as shown in the correct diagram.
- Reassemble the covers and add fresh, correctly mixed fuel.
- Check for leaks before starting. If fuel appears outside the tank, lines, or carburetor, stop and correct the fault first.
Do not turn carburetor screws to compensate for a primer that will not work. Carburetor adjustment will not repair a cracked bulb, blocked line, or failed purge passage. Use model-specific service data and a qualified technician for carburetor or speed adjustments.
When the bulb still will not prime
If the bulb and visible lines look sound but the bulb still will not move fuel, the fault may be inside the carburetor’s purge circuit. Small passages or check valves can block or fail. At this point, forcing the bulb or replacing parts at random may create more problems than it solves.
One common repair pattern is a bulb that has gone hard or split. Replacing it with the correct part may solve the leak, provided the lines and carburetor circuit are clear. Another pattern is an intact bulb with a blocked pickup or purge passage; a new bulb alone will not clear that obstruction. These are diagnostic examples, not reports of a specific repair.
Have a qualified service shop inspect the carburetor, pickup, or tank vent if the cause remains unclear. Stop and get help if fuel leaks, if connections will not seat, or if your unit’s parts diagram does not match the parts in front of you.
If the bulb now primes but the engine still will not start, move on to the cold- or warm-start steps in your manual. Check the air filter and other starting systems separately. A successful prime does not prove that the ignition, exhaust, or carburetor is ready for the engine to run.
Store the FS 38 to reduce repeat problems
Fuel that sits too long can lose quality, and ethanol-related fuel damage can affect small-engine parts. Follow Stihl’s storage instructions for draining or running the fuel system, or for using approved premixed fuel. Before the next season, inspect the bulb and lines for cracks, swelling, brittleness, and leaks.
Keep the trimmer in a cool, dry place away from flames. Clean the air filter and inspect the tank pickup and fuel lines at the intervals in the manual. The FS 38 has a line head, not a chain or bar; check the head markings or manual for the correct line size and cutting width.
Frequently asked questions
How many times should I press the FS 38 primer bulb?
Press it at least five times before starting, even if fuel is visible in the bulb.
Why will the primer bulb not fill?
A cracked bulb, loose or incorrectly routed line, air leak, blocked filter, or carburetor purge fault may stop it from filling.
Why does the bulb stay collapsed?
Check for a blocked pickup, kinked or restricted line, or restricted tank vent. Do not force the bulb.
Will replacing the bulb always fix the problem?
No. A blocked fuel line, pickup filter, purge passage, or failed carburetor valve can prevent priming even with a new bulb.
Can I fit a universal primer bulb?
Use only parts confirmed for your FS 38 serial number and carburetor version. Some versions may require a specified carburetor assembly.
Should I keep pulling the starter if the bulb will not prime?
No. Repeated pulls will not clear a failed purge circuit. Find the fuel fault before trying to start the engine again.
What fuel mix does the FS 38 use?
The specified mix is 50:1 gasoline to Stihl two-cycle oil. Follow the fuel requirements on your unit’s manual or label.
Can I run the engine to check for a fuel leak?
No. Check for leaks before starting. If fuel escapes, switch off, let the engine cool, and repair the leak first.
What if the bulb primes but the trimmer will not start?
Use the correct cold- or warm-start procedure, then check ignition, air filtration, exhaust, and carburetion separately.
Should I adjust the carburetor screws to make the bulb work?
No. Carburetor screws do not repair a damaged bulb, blocked line, or failed purge circuit. Use model-specific service guidance for adjustments.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
