If your Stihl FS 38 won’t start, stalls, loses power, or has string feeding problems, most issues can be diagnosed and repaired without replacing the entire trimmer.
Stihl FS 38 Grass Trimmer Problems
1. Stihl FS 38 Won’t Start
One of the most common issues is an engine that refuses to start.
Possible Causes
- Old or stale fuel
- Dirty carburetor
- Clogged fuel filter
- Fouled spark plug
- Dirty air filter
- Flooded engine
- Incorrect starting procedure
How to Fix It
- Drain old fuel and refill with fresh 50:1 fuel mixture.
- Replace a dirty fuel filter.
- Remove and clean or replace the spark plug.
- Clean or replace the air filter.
- Check the primer bulb for cracks or damage.
- Clean the carburetor if fuel has been left sitting for a long period.
If the engine becomes flooded, remove the spark plug, allow it to dry, pull the starter rope several times to clear excess fuel, reinstall the plug, and restart using the correct procedure.
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2. Engine Starts Then Dies
Sometimes the engine starts normally but shuts off after a few seconds.
Possible Causes
- Dirty carburetor
- Restricted fuel flow
- Clogged spark arrestor
- Blocked fuel tank vent
- Incorrect idle adjustment
How to Fix It
- Clean the carburetor thoroughly.
- Replace clogged fuel lines.
- Install a new fuel filter if necessary.
- Clean the spark arrestor screen.
- Check that the fuel tank vent is not blocked.
3. Loss of Power While Cutting
If the trimmer struggles when cutting thick grass or weeds, several components may require attention.
Possible Causes
- Dirty air filter
- Clogged exhaust
- Worn spark plug
- Fuel starvation
- Dirty carburetor
How to Fix It
- Clean or replace the air filter.
- Remove carbon buildup from the muffler screen.
- Replace the spark plug.
- Use fresh fuel.
- Clean the carburetor and fuel passages.
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4. Hard Starting When Cold
Cold starting problems often develop after the trimmer has been stored for an extended period.
Possible Causes
- Old fuel
- Dirty carburetor
- Weak spark
- Fuel evaporation
How to Fix It
- Drain stale fuel.
- Prime the engine correctly.
- Replace the spark plug if needed.
- Always use fresh fuel mixed at the proper ratio.
5. Engine Runs Only With the Choke On
If the engine stalls whenever the choke is opened, the fuel system is usually restricted.
Possible Causes
- Dirty carburetor jets
- Clogged fuel filter
- Cracked fuel line
- Air leak
How to Fix It
- Clean the carburetor.
- Replace damaged fuel lines.
- Install a new fuel filter.
- Inspect carburetor gaskets for leaks.
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6. Trimmer Line Won’t Feed
Line feeding problems are among the most common complaints from homeowners.
Possible Causes
- Incorrect line size
- Overfilled spool
- Worn bump head
- Damaged spring
- Incorrect line installation
How to Fix It
- Reload the spool correctly.
- Use the recommended trimmer line diameter.
- Tap the bump head firmly while operating at full throttle.
- Replace worn bump head components if necessary.
7. Excessive Vibration
Excessive vibration makes trimming uncomfortable and may indicate damaged components.
Possible Causes
- Bent driveshaft
- Damaged trimmer head
- Loose bolts
- Worn clutch
How to Fix It
- Tighten all loose hardware.
- Replace damaged cutting head components.
- Inspect the clutch for excessive wear.
- Replace a bent driveshaft if required.
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8. Fuel Leaks
Fuel leaks should be repaired immediately to prevent safety hazards.
Common Leak Points
- Fuel lines
- Primer bulb
- Fuel cap
- Carburetor gasket
How to Fix It
- Replace cracked fuel hoses.
- Install a new primer bulb if damaged.
- Replace worn fuel cap seals.
- Repair leaking carburetor gaskets.
Never operate a trimmer that is leaking fuel.
9. Poor Idle Performance
The engine may stall while idling, or the cutting head may continue spinning even at idle speed.
Possible Causes
- Dirty carburetor
- Incorrect idle adjustment
- Air leak
How to Fix It
- Clean the carburetor.
- Adjust the idle speed screw according to the owner’s manual.
- Replace damaged intake gaskets if air leaks are present.
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10. Excessive Exhaust Smoke
A small amount of smoke is normal for two-stroke engines, but heavy smoke usually indicates another issue.
Possible Causes
- Too much oil in the fuel mixture
- Incorrect fuel ratio
- Dirty air filter
How to Fix It
- Mix fuel using the recommended 50:1 ratio.
- Use quality two-stroke engine oil.
- Replace dirty air filters.
11. Poor Fuel Economy
If the trimmer consumes fuel unusually quickly, engine efficiency may have decreased.
Possible Causes
- Dirty air filter
- Carburetor adjustment problems
- Fuel leaks
- Worn spark plug
How to Fix It
- Replace the air filter.
- Tune or clean the carburetor.
- Repair any fuel leaks.
- Install a new spark plug.
12. Primer Bulb Won’t Fill
The primer bulb should draw fuel through the carburetor before starting.
Possible Causes
- Blocked fuel filter
- Cracked fuel lines
- Clogged fuel tank vent
- Dirty carburetor
How to Fix It
- Replace damaged fuel lines.
- Install a new fuel filter.
- Clean the carburetor.
- Inspect the tank vent for blockage.
Preventive Maintenance Tips
| Maintenance Item | Recommended Interval |
|---|---|
| Clean air filter | Every 10 operating hours |
| Inspect spark plug | Every 25 operating hours |
| Replace spark plug | Every 100 operating hours |
| Replace fuel filter | Once per year or as needed |
| Clean cooling fins | Every season |
| Drain fuel before long-term storage | Before storage |
| Inspect fuel lines | Every season |
| Clean trimmer head | After every use |
When to Seek Professional Repair
While many FS 38 problems can be fixed at home, some repairs require specialized tools or experience. Consider professional service if:
- The engine has very low compression.
- Crankshaft seals are leaking.
- The ignition module has failed.
- The clutch assembly is severely worn.
- The driveshaft is bent or damaged.
- Carburetor cleaning and basic maintenance do not solve the problem.
Final Thoughts
The Stihl FS 38 is a dependable and easy-to-maintain grass trimmer that delivers excellent performance for routine lawn care. Most common problems, including hard starting, stalling, poor power, fuel leaks, and line feeding issues, are caused by stale fuel, clogged carburetors, dirty filters, or worn maintenance parts.
Performing regular maintenance, using fresh 50:1 fuel, cleaning the air filter, replacing the spark plug when needed, and inspecting the fuel system can prevent many of these issues. With proper care, the Stihl FS 38 can provide years of reliable trimming performance.
