Stihl FS 38 Carburetor Settings (L & H Specs)
There is no safe, universal L- or H-screw turns-out setting for every STIHL FS 38. Carburetor versions and emissions settings can differ, and a lean high-speed setting can damage the engine. Identify your carburetor and check the manual for your exact model before adjusting it. The listed idle-speed reference is 2,800 rpm; high-speed tuning requires the specified service procedure.
If your trimmer starts poorly, surges, or stalls, turning carburetor screws may seem like the fastest fix. But the same symptoms can come from stale fuel, a blocked filter, a damaged fuel line, or an air leak. Adjusting the carb before checking those causes can hide a fault and put the engine at risk.
I use a simple order when diagnosing an FS 38: confirm the exact version, inspect the fuel and air path, then make only adjustments allowed by the matching STIHL service information. That approach helps separate a genuine tuning problem from a maintenance fault.
Confirm the FS 38 version before adjusting
The FS 38 is a 27.2 cm³, two-stroke grass trimmer. Its carburetor and emissions setup may vary by model version or market, so an adjustment value found online may not match your machine. Use the model identification and serial number to locate the correct operator’s manual or service information before touching the carburetor.
The carburetor’s L and H screws control fuel delivery at different engine speeds. L affects the low-speed circuit, including pickup from idle; H affects the high-speed circuit. These are not universal “set and forget” screws: their correct positions depend on the carburetor and the manufacturer’s procedure.
Check the identification label on the trimmer and compare it with the manual. If the carburetor has limiter caps, restricted screws, or markings that do not match the instructions you found, do not force or remove anything. A STIHL dealer can identify the carburetor and access the right service information.
| Item | Verified reference | How to use it |
|---|---|---|
| Engine displacement | 27.2 cm³ | Confirms the FS 38 engine class |
| Fuel mix with STIHL two-cycle oil | 50:1 | Use fresh fuel mixed as directed |
| Idle-speed reference | 2,800 rpm | Check against the exact manual and service procedure |
| Maximum engine-speed reference | 9,500 rpm | Not a target to exceed or set by ear |
These figures do not replace the manual for your serial-number version. In particular, the maximum-speed reference is not permission to tune the H screw until the engine sounds fastest.
Prepare the trimmer and work area safely
Carburetor work begins with a safe, serviceable machine, not with a screwdriver. Inspect the FS 38 while it is off and cool. Fix fuel leaks, damaged parts, or control problems before starting it, and keep the cutting attachment away from people and the ground during any running check.
Before you start, inspect:
- Fuel lines and tank for cracks, leaks, or loose connections.
- Air filter for dirt, damage, or poor fit.
- Spark plug for wear or fouling, following the manual’s service guidance.
- Cutting attachment and deflector for correct fit and damage.
- Throttle and stop controls for proper movement and return.
- Spark-arresting screen for blockage, if fitted and serviceable on your version.
Use eye or face protection, hearing protection, gloves, long leg protection, and sturdy footwear. Clear people, pets, and loose debris from the work area. Use only an approved cutting attachment with its matching deflector.
For a running check, follow the starting and warm-up steps in the operator’s manual. Keep a firm grip and make sure the attachment cannot contact the ground, tools, or bystanders. If the attachment moves at idle, stop and correct the problem before trimming; the clutch or idle setting may need inspection.
Rule out fuel and air problems first
A carburetor adjustment cannot reliably repair a blocked filter, poor fuel, or an intake leak. These faults can make the engine act lean or unstable even when the screw positions are correct. Inspect the simple causes first, then consider tuning only if the symptoms remain and the service information permits adjustment.
Start with the fuel. Use fresh fuel mixed at the specified 50:1 ratio with STIHL two-cycle oil, or follow the exact instructions for the oil and fuel specified in your manual. Do not use straight gasoline. Fuel that has been stored too long or mixed incorrectly can cause hard starting, poor acceleration, or rough running.
Next, check the air filter and fuel filter using the manual’s service steps. A dirty air filter can affect how the engine runs, while restricted fuel flow may cause power loss or stalling. Inspect fuel lines for damage and look for signs of air leakage around the intake. If you find a leak or a damaged line, repair it before running the trimmer.
| Symptom | Check before carburetor adjustment | Next step |
|---|---|---|
| Hard starting | Fresh 50:1 mix, choke procedure, plug condition | Follow the manual’s start and service checks |
| Stalls at idle | Air filter, fuel condition, idle behavior | Use only the approved idle procedure |
| Hesitates when throttle opens | Fuel filter, fuel lines, intake leaks | Repair faults before considering L-circuit service |
| Loses power at full throttle | Fuel flow, air filter, spark-arresting screen | Stop if power loss continues; seek service |
| Attachment turns at idle | Idle speed and clutch condition | Stop operation until corrected |
This order is useful because a screw change may briefly alter a symptom without fixing its cause. If the engine surges, runs unusually hot, or loses power, stop using it. A lean condition from incorrect adjustment, an air leak, or restricted fuel delivery can overheat the engine and cause piston seizure.
What to do with the L and H settings
The correct L and H specifications are the values and procedure in the STIHL service information for the exact carburetor fitted to your FS 38. There is no safe universal number of turns out to provide here. Do not use a generic internet setting, and do not tune the H screw by ear.
Some carburetor versions may have restricted or capped adjustment screws. Do not remove a limiter or force a screw to reach a position copied from another model. If your manual does not provide an adjustment procedure, or the settings are restricted, have an authorized STIHL dealer inspect and tune the unit.
The H circuit requires special care. A lean high-speed setting can raise engine temperature and damage the piston and cylinder; an overly rich setting can cause poor running and plug fouling. A tachometer and the specified service procedure are needed where high-speed adjustment is allowed. Do not exceed the stated maximum engine-speed reference of 9,500 rpm, and do not treat that figure as a tuning target.
The idle reference is 2,800 rpm, but a speed figure alone is not a complete adjustment procedure. The cutting attachment must remain stationary at idle. If the engine cannot idle properly without attachment movement, stop and have the carburetor or clutch checked rather than trying to compensate by guesswork.
Diagnostic examples: choosing the right next step
These examples show how I separate a tuning complaint from a fuel-system fault. They are diagnostic patterns, not claims about a particular serial-numbered trimmer. In each case, the manual for the exact FS 38 version remains the authority for service steps and adjustment limits.
Example: it starts, then stalls when you squeeze the throttle. First check fuel freshness, the air filter, and the fuel filter. Inspect lines for damage or leakage. If those items are sound and the symptom remains, consult the correct service information before any L-circuit work. Do not make repeated screw changes to cover a suspected restriction.
Example: it runs fast but the attachment spins at idle. Stop the engine and check the idle procedure in the manual. The attachment must stay still at idle. If it continues to move after the permitted idle adjustment, or the engine races, stop operation and have the clutch and carburetor inspected.
Example: it bogs at full throttle or seems unusually hot. Do not turn the H screw until the sound changes. Check for blocked fuel delivery, a dirty filter, a restricted spark-arresting screen, or an air leak. Continued high-speed use with a lean condition can cause severe engine damage, so stop if the symptom persists.
Building on this, record what you observe before service: fuel age and mix, whether the engine starts cold, how it idles, and when it loses power. Clear notes help a dealer reproduce the fault without relying on a guessed screw position.
After adjustment and seasonal storage
A correct repair should leave the trimmer starting and running according to its manual, with stable idle and a stationary cutting attachment. After any authorized adjustment, check operation in a clear area and stop if you notice fuel leakage, abnormal noise, vibration, overheating, or loss of power.
Keep the air filter and spark-arresting screen clean, and service the spark plug and fuel filter at the intervals in the manual. Before extended storage, follow STIHL’s fuel-system instructions for your version. Store the trimmer cool and dry, away from ignition sources, with the cutting attachment guarded. When returning it to service, use fresh, correctly mixed 50:1 fuel.
The main takeaway is simple: verify the carburetor version, correct maintenance faults first, and never guess at L or H settings. If the machine needs high-speed tuning or has restricted screws, use an authorized dealer and the specified procedure.
Frequently asked questions
What are the STIHL FS 38 L and H screw settings?
There is no universal turns-out setting that safely applies to every FS 38. Use the service information for the exact carburetor and model version.
What is the FS 38 idle speed?
The listed idle-speed reference is 2,800 rpm. Confirm the correct procedure in the manual for your exact version, and ensure the cutting attachment stays still at idle.
What is the maximum engine speed?
The stated reference is 9,500 rpm. It is not a target to exceed, and H-circuit tuning should not be done by ear.
Can I adjust the H screw by listening to the engine?
No. Use the specified service procedure and tachometer where adjustment is permitted. Incorrect high-speed tuning can overheat and damage the engine.
Why does my FS 38 stall when I open the throttle?
Stale or incorrectly mixed fuel, blocked filters, damaged fuel lines, or an air leak can cause this symptom. Check those faults before considering carburetor adjustment.
What fuel mix does the FS 38 use?
The reference mix with STIHL two-cycle oil is 50:1. Use fresh fuel and follow the instructions in your exact operator’s manual.
Can I use a carburetor setting from another STIHL trimmer?
No. Carburetor design and emissions configuration can differ by model and market. A setting for another machine may be unsafe for your FS 38.
Why does the cutting attachment spin at idle?
The idle speed may be wrong, or the clutch may need inspection. Stop using the trimmer until the attachment remains stationary at idle.
Should I remove limiter caps to adjust the carburetor?
Do not remove or force restricted parts. If the correct service procedure is not available or adjustment is limited, contact an authorized STIHL dealer.
Can carburetor adjustment fix an air leak or blocked fuel filter?
No. Repair the leak or restriction first. Screw changes cannot reliably correct a fuel-delivery or intake fault, and may make engine damage more likely.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
