Stihl FS 38 Carburetor Adjustment (Step-by-Step)
On an STIHL FS 38, adjust the LA screw to set idle speed; do not use it to alter the fuel mixture. Warm the 27.2 cm³ engine, then turn LA in small steps: clockwise raises idle, while counterclockwise lowers it. The cutting attachment must stay still at idle. If it won’t, or the engine races or stalls, stop and seek service.
Imagine the trimmer starts after sitting all winter, but its head spins while you’re letting it idle. Is the carburetor out of tune, or is something else wrong? That distinction matters: an idle-speed adjustment may help, but changing a mixture screw without the correct carburetor instructions can damage the engine.
I treat LA adjustment as a small, controlled correction, not a cure for every starting or running problem. The FS 38 has a 27.2 cm³ two-stroke engine. Its fuel must be mixed at 50:1 with STIHL two-stroke oil, or an equivalent oil that meets the manual’s specification. Carburetor details may vary by production version, so check the machine’s manual and carburetor markings before touching any screw.
Equipment Setup and Safety Preparation
This section covers the checks and safe setup to complete before changing idle speed. A steady idle test requires a warm engine, a clear work area, and a cutting attachment that is right for the specific FS 38 configuration. It also requires knowing which screw is LA, rather than guessing at a mixture screw.
Check the trimmer before starting
A carburetor adjustment changes idle speed. It does not fix stale or incorrectly mixed fuel, restricted airflow, a damaged throttle control, or an ignition fault. Check the manual for the correct starting procedure and for the approved cutting attachment and guard for your model.
- Wear eye or face protection, hearing protection, gloves, long trousers, and sturdy footwear.
- Inspect the throttle control, stop switch, guard, harness, and cutting attachment for damage or loose parts.
- Clear debris from the work area. Keep people at least 15 m (50 ft) away.
- Place the trimmer on stable ground with the attachment clear of people, objects, and the ground.
- Use fresh fuel mixed at 50:1. Do not use straight gasoline.
Before turning anything, identify LA using the markings and instructions for your carburetor version. Do not assume that H and L screws are adjustable. Their presence and permitted settings depend on the carburetor fitted to the machine. There is no safe universal H/L setting for every FS 38.
Warm the engine and establish a baseline
Start the machine according to its cold- or warm-start instructions. Let it reach normal operating temperature before setting idle; a cold engine may not idle the same way. Keep hands, clothing, and tools clear of the cutting attachment throughout the test.
Note what the trimmer does before adjustment. Does it stall at idle? Does the attachment rotate? Does the engine sound unusually fast? This baseline helps you judge whether a small LA change improves the idle or points to a fault that needs inspection.
Field Trimming Operation
This section explains the adjustment sequence and what to check afterward. The target is not simply an engine that stays running: the engine should idle steadily while the attachment remains stationary, then respond cleanly to the throttle. If it cannot meet both conditions, stop rather than forcing the adjustment.
Adjust the LA idle-speed screw
LA is the idle-speed adjustment. Turning it clockwise slightly raises idle speed; turning it counterclockwise lowers idle speed. Make small changes, then pause to observe the result. The FS 38 specification lists an idle speed of approximately 2,800 rpm; use a tachometer and the applicable manual for an exact service limit.
- With the engine warm and idling, locate LA as shown in the manual for your carburetor.
- If the engine stalls or idles roughly, turn LA clockwise a small amount to raise idle.
- If the cutting attachment rotates at idle, turn LA counterclockwise in small steps until it stops, while keeping the engine running steadily.
- After each change, wait briefly and check both engine idle and attachment movement.
- Stop adjusting if the engine races, the attachment keeps turning, or a small correction does not produce a stable result.
Do not turn H or L to stop the attachment. Those screws affect the fuel mixture, not the safe correction for an attachment that rotates at idle. Incorrect mixture settings can cause excessive engine speed and inadequate lubrication. In severe cases, the piston and cylinder can seize.
Verify throttle response
Once the attachment stays still at idle, test the throttle briefly in a clear area. The engine should pick up cleanly, then return to a steady idle when you release the throttle. Keep the attachment clear of the ground and anything it could strike during this check.
| What you observe | What to do |
|---|---|
| Engine idles steadily; attachment stays still | Briefly check throttle response, then confirm it returns to stable idle. |
| Engine stalls or idles roughly | Raise idle slightly with LA. If it persists, stop and inspect the fuel and machine condition. |
| Attachment turns at idle | Lower idle slightly with LA. If it still turns, stop using the trimmer and arrange inspection. |
| Engine races, overheats, or loses power | Stop the engine. Do not try to correct this by tuning H or L. |
| LA changes do not produce stable idle | Stop and have the carburetor and drive system inspected by a STIHL service dealer. |
A rotating attachment is not a condition to work around. If it continues to move after careful LA adjustment, shut the machine off. The cause may need diagnosis beyond idle speed, and continued operation can create a safety risk.
Maintenance, Carburetor Adjustment, and Cutting-Attachment Care
This section separates a simple idle correction from problems that need repair. A carburetor can be affected by fuel or airflow problems, while a rotating attachment can also point to a drive-system issue. Adjustment is appropriate only when the machine is otherwise in sound condition.
Use symptoms to choose the next step
The examples below are diagnostic scenarios, not claims that every FS 38 has the same fault. They show why I check the symptom before turning a screw: a single adjustment can mask a problem without fixing its cause.
| Scenario | Reasonable next step |
|---|---|
| The warm engine stalls at idle, but runs smoothly when the throttle is pressed | Make a small clockwise LA adjustment. If the idle remains unstable, stop and inspect or seek service. |
| The engine idles but the attachment keeps moving | Turn LA counterclockwise slightly. If movement continues, shut down and have the machine inspected. |
| The trimmer runs poorly after stored fuel was used | Stop and use fresh fuel mixed at 50:1. Follow the manual for draining or servicing the fuel system; do not compensate by changing H/L settings. |
| The engine races or loses power after a screw was turned | Stop operation. Restore settings only by following carburetor-specific service instructions, or have a dealer check it. |
| The engine will not hold idle after basic checks | Request inspection of the carburetor and related systems rather than repeatedly adjusting LA. |
A blocked fuel filter, air leak, restricted exhaust, or other fault may cause poor running. Changing mixture settings to compensate can make the engine run too fast or too lean, which can reduce lubrication and damage the engine. Stop if the trimmer races, overheats, or loses power.
Keep the cutting attachment and guard matched to the specific FS 38 configuration. Do not use chainsaw specifications for this trimmer: it has no guide bar, chain, chain-oil system, or wood-hardness rating.
Shutdown, Cleaning, and Seasonal Storage
This section covers the final safety check and care after adjustment. A successful idle setting should hold only when the engine is warm and the machine is in sound condition. Shutting down, cooling the trimmer, and following the storage instructions can help you avoid repeating the same troubleshooting next season.
After the throttle-response check, release the control and confirm the attachment stops at idle. Switch off the engine, let the machine cool, and clean it as directed in the manual. If it stalls repeatedly, the attachment continues rotating, or you cannot get a stable idle, leave further carburetor work to a STIHL service dealer.
For seasonal storage, follow the FS 38 manual’s instructions for fuel and the fuel system. Do not leave old fuel in the machine on the assumption that an LA adjustment will fix a later starting problem. Before the next use, check the fuel mix, guard, attachment, throttle, and stop switch again.
Frequently asked questions
Which screw adjusts idle speed on an FS 38?
LA adjusts idle speed. Confirm its location in the manual for your carburetor version.
Which way do I turn LA to raise idle?
Turn LA clockwise in a small step. Turn it counterclockwise to lower idle.
What is the FS 38 idle speed?
The listed idle speed is approximately 2,800 rpm. Use a tachometer and the applicable manual if you need an exact service limit.
Should the cutting attachment spin at idle?
No. It should remain stationary while the warm engine idles steadily.
Can I adjust H and L to stop the head from spinning?
No. Do not use mixture screws to stop attachment movement. Adjust LA; if movement continues, stop and seek service.
Can I use one H/L setting for every FS 38?
No. Carburetor versions and permitted settings can vary. Follow instructions for the exact carburetor and use the required service tools.
What fuel mix does the FS 38 use?
Use a 50:1 gasoline-to-two-stroke-oil mix with STIHL oil or an equivalent oil meeting the manual’s specification.
What if the engine still stalls after adjusting LA?
Stop repeated adjustments. Check for fresh, correctly mixed fuel and follow the manual’s maintenance checks. Persistent stalling needs further diagnosis.
What should I do if the engine races or overheats?
Stop the engine and do not try to tune it by ear or alter H/L screws. Arrange inspection before using it again.
When should a dealer inspect the trimmer?
Seek service if it repeatedly stalls, races, overheats, loses power, or keeps turning the attachment at idle after a careful LA adjustment.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
